


PERLSUB(1)       Perl Programmers Reference Guide      PERLSUB(1)


NAME
       perlsub - Perl subroutines

SYNOPSIS
       To declare subroutines:

           sub NAME;                     # A "forward" declaration.
           sub NAME(PROTO);              #  ditto, but with prototypes
           sub NAME : ATTRS;             #  with attributes
           sub NAME(PROTO) : ATTRS;      #  with attributes and prototypes

           sub NAME BLOCK                # A declaration and a definition.
           sub NAME(PROTO) BLOCK         #  ditto, but with prototypes
           sub NAME : ATTRS BLOCK        #  with attributes
           sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes

       To define an anonymous subroutine at runtime:

           $subref = sub BLOCK;                 # no proto
           $subref = sub (PROTO) BLOCK;         # with proto
           $subref = sub : ATTRS BLOCK;         # with attributes
           $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes

       To import subroutines:

           use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

           NAME(LIST);    # & is optional with parentheses.
           NAME LIST;     # Parentheses optional if predeclared/imported.
           &NAME(LIST);   # Circumvent prototypes.
           &NAME;         # Makes current @_ visible to called subroutine.

DESCRIPTION
       Like many languages, Perl provides for user-defined
       subroutines.  These may be located anywhere in the main
       program, loaded in from other files via the "do",
       "require", or "use" keywords, or generated on the fly
       using "eval" or anonymous subroutines.  You can even call
       a function indirectly using a variable containing its name
       or a CODE reference.

       The Perl model for function call and return values is
       simple: all functions are passed as parameters one single
       flat list of scalars, and all functions likewise return to
       their caller one single flat list of scalars.  Any arrays
       or hashes in these call and return lists will collapse,
       losing their identities--but you may always use pass-by-
       reference instead to avoid this.  Both call and return
       lists may contain as many or as few scalar elements as
       you'd like.  (Often a function without an explicit return
       statement is called a subroutine, but there's really no
       difference from Perl's perspective.)



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       Any arguments passed in show up in the array @_.
       Therefore, if you called a function with two arguments,
       those would be stored in $_[0] and $_[1].  The array @_ is
       a local array, but its elements are aliases for the actual
       scalar parameters.  In particular, if an element $_[0] is
       updated, the corresponding argument is updated (or an
       error occurs if it is not updatable).  If an argument is
       an array or hash element which did not exist when the
       function was called, that element is created only when
       (and if) it is modified or a reference to it is taken.
       (Some earlier versions of Perl created the element whether
       or not the element was assigned to.)  Assigning to the
       whole array @_ removes that aliasing, and does not update
       any arguments.

       A "return" statement may be used to exit a subroutine,
       optionally specifying the returned value, which will be
       evaluated in the appropriate context (list, scalar, or
       void) depending on the context of the subroutine call.  If
       you specify no return value, the subroutine returns an
       empty list in list context, the undefined value in scalar
       context, or nothing in void context.  If you return one or
       more aggregates (arrays and hashes), these will be
       flattened together into one large indistinguishable list.

       If no "return" is found and if the last statement is an
       expression, its value is returned. If the last statement
       is a loop control structure like a "foreach" or a "while",
       the returned value is unspecified. The empty sub returns
       the empty list.

       Perl does not have named formal parameters.  In practice
       all you do is assign to a "my()" list of these.  Variables
       that aren't declared to be private are global variables.
       For gory details on creating private variables, see
       "Private Variables via my()" and "Temporary Values via
       local()".  To create protected environments for a set of
       functions in a separate package (and probably a separate
       file), see "Packages" in perlmod.

       Example:

           sub max {
               my $max = shift(@_);
               foreach $foo (@_) {
                   $max = $foo if $max < $foo;
               }
               return $max;
           }
           $bestday = max($mon,$tue,$wed,$thu,$fri);

       Example:





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           # get a line, combining continuation lines
           #  that start with whitespace

           sub get_line {
               $thisline = $lookahead;  # global variables!
               LINE: while (defined($lookahead = <STDIN>)) {
                   if ($lookahead =~ /^[ \t]/) {
                       $thisline .= $lookahead;
                   }
                   else {
                       last LINE;
                   }
               }
               return $thisline;
           }

           $lookahead = <STDIN>;       # get first line
           while (defined($line = get_line())) {
               ...
           }

       Assigning to a list of private variables to name your
       arguments:

           sub maybeset {
               my($key, $value) = @_;
               $Foo{$key} = $value unless $Foo{$key};
           }

       Because the assignment copies the values, this also has
       the effect of turning call-by-reference into call-by-
       value.  Otherwise a function is free to do in-place
       modifications of @_ and change its caller's values.

           upcase_in($v1, $v2);  # this changes $v1 and $v2
           sub upcase_in {
               for (@_) { tr/a-z/A-Z/ }
           }

       You aren't allowed to modify constants in this way, of
       course.  If an argument were actually literal and you
       tried to change it, you'd take a (presumably fatal)
       exception.   For example, this won't work:

           upcase_in("frederick");

       It would be much safer if the "upcase_in()" function were
       written to return a copy of its parameters instead of
       changing them in place:








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           ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
           sub upcase {
               return unless defined wantarray;  # void context, do nothing
               my @parms = @_;
               for (@parms) { tr/a-z/A-Z/ }
               return wantarray ? @parms : $parms[0];
           }

       Notice how this (unprototyped) function doesn't care
       whether it was passed real scalars or arrays.  Perl sees
       all arguments as one big, long, flat parameter list in @_.
       This is one area where Perl's simple argument-passing
       style shines.  The "upcase()" function would work
       perfectly well without changing the "upcase()" definition
       even if we fed it things like this:

           @newlist   = upcase(@list1, @list2);
           @newlist   = upcase( split /:/, $var );

       Do not, however, be tempted to do this:

           (@a, @b)   = upcase(@list1, @list2);

       Like the flattened incoming parameter list, the return
       list is also flattened on return.  So all you have managed
       to do here is stored everything in @a and made @b empty.
       See "Pass by Reference" for alternatives.

       A subroutine may be called using an explicit "&" prefix.
       The "&" is optional in modern Perl, as are parentheses if
       the subroutine has been predeclared.  The "&" is not
       optional when just naming the subroutine, such as when
       it's used as an argument to defined() or undef().  Nor is
       it optional when you want to do an indirect subroutine
       call with a subroutine name or reference using the
       "&$subref()" or "&{$subref}()" constructs, although the
       "$subref->()" notation solves that problem.  See perlref
       for more about all that.

       Subroutines may be called recursively.  If a subroutine is
       called using the "&" form, the argument list is optional,
       and if omitted, no @_ array is set up for the subroutine:
       the @_ array at the time of the call is visible to
       subroutine instead.  This is an efficiency mechanism that
       new users may wish to avoid.

           &foo(1,2,3);        # pass three arguments
           foo(1,2,3);         # the same

           foo();              # pass a null list
           &foo();             # the same

           &foo;               # foo() get current args, like foo(@_) !!
           foo;                # like foo() IFF sub foo predeclared, else "foo"



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       Not only does the "&" form make the argument list
       optional, it also disables any prototype checking on
       arguments you do provide.  This is partly for historical
       reasons, and partly for having a convenient way to cheat
       if you know what you're doing.  See Prototypes below.

       Subroutines whose names are in all upper case are reserved
       to the Perl core, as are modules whose names are in all
       lower case.  A subroutine in all capitals is a loosely-
       held convention meaning it will be called indirectly by
       the run-time system itself, usually due to a triggered
       event.  Subroutines that do special, pre-defined things
       include "AUTOLOAD", "CLONE", "DESTROY" plus all functions
       mentioned in perltie and PerlIO::via.

       The "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END"
       subroutines are not so much subroutines as named special
       code blocks, of which you can have more than one in a
       package, and which you can not call explicitly.  See
       "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod

   Private Variables via my()
       Synopsis:

           my $foo;            # declare $foo lexically local
           my (@wid, %get);    # declare list of variables local
           my $foo = "flurp";  # declare $foo lexical, and init it
           my @oof = @bar;     # declare @oof lexical, and init it
           my $x : Foo = $y;   # similar, with an attribute applied

       WARNING: The use of attribute lists on "my" declarations
       is still evolving.  The current semantics and interface
       are subject to change.  See attributes and
       Attribute::Handlers.

       The "my" operator declares the listed variables to be
       lexically confined to the enclosing block, conditional
       ("if/unless/elsif/else"), loop
       ("for/foreach/while/until/continue"), subroutine, "eval",
       or "do/require/use"'d file.  If more than one value is
       listed, the list must be placed in parentheses.  All
       listed elements must be legal lvalues.  Only alphanumeric
       identifiers may be lexically scoped--magical built-ins
       like $/ must currently be "local"ized with "local"
       instead.

       Unlike dynamic variables created by the "local" operator,
       lexical variables declared with "my" are totally hidden
       from the outside world, including any called subroutines.
       This is true if it's the same subroutine called from
       itself or elsewhere--every call gets its own copy.

       This doesn't mean that a "my" variable declared in a
       statically enclosing lexical scope would be invisible.



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       Only dynamic scopes are cut off.   For example, the
       "bumpx()" function below has access to the lexical $x
       variable because both the "my" and the "sub" occurred at
       the same scope, presumably file scope.

           my $x = 10;
           sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the
       scope it is being evaluated in, so long as the names
       aren't hidden by declarations within the "eval()" itself.
       See perlref.

       The parameter list to my() may be assigned to if desired,
       which allows you to initialize your variables.  (If no
       initializer is given for a particular variable, it is
       created with the undefined value.)  Commonly this is used
       to name input parameters to a subroutine.  Examples:

           $arg = "fred";        # "global" variable
           $n = cube_root(27);
           print "$arg thinks the root is $n\n";
        fred thinks the root is 3

           sub cube_root {
               my $arg = shift;  # name doesn't matter
               $arg **= 1/3;
               return $arg;
           }

       The "my" is simply a modifier on something you might
       assign to.  So when you do assign to variables in its
       argument list, "my" doesn't change whether those variables
       are viewed as a scalar or an array.  So

           my ($foo) = <STDIN>;                # WRONG?
           my @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           my $foo = <STDIN>;

       supplies a scalar context.  But the following declares
       only one variable:

           my $foo, $bar = 1;                  # WRONG

       That has the same effect as

           my $foo;
           $bar = 1;

       The declared variable is not introduced (is not visible)
       until after the current statement.  Thus,



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           my $x = $x;

       can be used to initialize a new $x with the value of the
       old $x, and the expression

           my $x = 123 and $x == 123

       is false unless the old $x happened to have the value 123.

       Lexical scopes of control structures are not bounded
       precisely by the braces that delimit their controlled
       blocks; control expressions are part of that scope, too.
       Thus in the loop

           while (my $line = <>) {
               $line = lc $line;
           } continue {
               print $line;
           }

       the scope of $line extends from its declaration throughout
       the rest of the loop construct (including the "continue"
       clause), but not beyond it.  Similarly, in the conditional

           if ((my $answer = <STDIN>) =~ /^yes$/i) {
               user_agrees();
           } elsif ($answer =~ /^no$/i) {
               user_disagrees();
           } else {
               chomp $answer;
               die "'$answer' is neither 'yes' nor 'no'";
           }

       the scope of $answer extends from its declaration through
       the rest of that conditional, including any "elsif" and
       "else" clauses, but not beyond it.  See "Simple
       statements" in perlsyn for information on the scope of
       variables in statements with modifiers.

       The "foreach" loop defaults to scoping its index variable
       dynamically in the manner of "local".  However, if the
       index variable is prefixed with the keyword "my", or if
       there is already a lexical by that name in scope, then a
       new lexical is created instead.  Thus in the loop

           for my $i (1, 2, 3) {
               some_function();
           }

       the scope of $i extends to the end of the loop, but not
       beyond it, rendering the value of $i inaccessible within
       "some_function()".

       Some users may wish to encourage the use of lexically



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       scoped variables.  As an aid to catching implicit uses to
       package variables, which are always global, if you say

           use strict 'vars';

       then any variable mentioned from there to the end of the
       enclosing block must either refer to a lexical variable,
       be predeclared via "our" or "use vars", or else must be
       fully qualified with the package name.  A compilation
       error results otherwise.  An inner block may countermand
       this with "no strict 'vars'".

       A "my" has both a compile-time and a run-time effect.  At
       compile time, the compiler takes notice of it.  The
       principal usefulness of this is to quiet "use strict
       'vars'", but it is also essential for generation of
       closures as detailed in perlref.  Actual initialization is
       delayed until run time, though, so it gets executed at the
       appropriate time, such as each time through a loop, for
       example.

       Variables declared with "my" are not part of any package
       and are therefore never fully qualified with the package
       name.  In particular, you're not allowed to try to make a
       package variable (or other global) lexical:

           my $pack::var;      # ERROR!  Illegal syntax

       In fact, a dynamic variable (also known as package or
       global variables) are still accessible using the fully
       qualified "::" notation even while a lexical of the same
       name is also visible:

           package main;
           local $x = 10;
           my    $x = 20;
           print "$x and $::x\n";

       That will print out 20 and 10.

       You may declare "my" variables at the outermost scope of a
       file to hide any such identifiers from the world outside
       that file.  This is similar in spirit to C's static
       variables when they are used at the file level.  To do
       this with a subroutine requires the use of a closure (an
       anonymous function that accesses enclosing lexicals).  If
       you want to create a private subroutine that cannot be
       called from outside that block, it can declare a lexical
       variable containing an anonymous sub reference:

           my $secret_version = '1.001-beta';
           my $secret_sub = sub { print $secret_version };
           &$secret_sub();




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       As long as the reference is never returned by any function
       within the module, no outside module can see the
       subroutine, because its name is not in any package's
       symbol table.  Remember that it's not REALLY called
       $some_pack::secret_version or anything; it's just
       $secret_version, unqualified and unqualifiable.

       This does not work with object methods, however; all
       object methods have to be in the symbol table of some
       package to be found.  See "Function Templates" in perlref
       for something of a work-around to this.

   Persistent Private Variables
       There are two ways to build persistent private variables
       in Perl 5.10.  First, you can simply use the "state"
       feature. Or, you can use closures, if you want to stay
       compatible with releases older than 5.10.

       Persistent variables via state()

       Beginning with perl 5.9.4, you can declare variables with
       the "state" keyword in place of "my". For that to work,
       though, you must have enabled that feature beforehand,
       either by using the "feature" pragma, or by using "-E" on
       one-liners. (see feature)

       For example, the following code maintains a private
       counter, incremented each time the gimme_another()
       function is called:

           use feature 'state';
           sub gimme_another { state $x; return ++$x }

       Also, since $x is lexical, it can't be reached or modified
       by any Perl code outside.

       When combined with variable declaration, simple scalar
       assignment to "state" variables (as in "state $x = 42") is
       executed only the first time.  When such statements are
       evaluated subsequent times, the assignment is ignored.
       The behavior of this sort of assignment to non-scalar
       variables is undefined.

       Persistent variables with closures

       Just because a lexical variable is lexically (also called
       statically) scoped to its enclosing block, "eval", or "do"
       FILE, this doesn't mean that within a function it works
       like a C static.  It normally works more like a C auto,
       but with implicit garbage collection.

       Unlike local variables in C or C++, Perl's lexical
       variables don't necessarily get recycled just because
       their scope has exited.  If something more permanent is



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       still aware of the lexical, it will stick around.  So long
       as something else references a lexical, that lexical won't
       be freed--which is as it should be.  You wouldn't want
       memory being free until you were done using it, or kept
       around once you were done.  Automatic garbage collection
       takes care of this for you.

       This means that you can pass back or save away references
       to lexical variables, whereas to return a pointer to a C
       auto is a grave error.  It also gives us a way to simulate
       C's function statics.  Here's a mechanism for giving a
       function private variables with both lexical scoping and a
       static lifetime.  If you do want to create something like
       C's static variables, just enclose the whole function in
       an extra block, and put the static variable outside the
       function but in the block.

           {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
               }
           }
           # $secret_val now becomes unreachable by the outside
           # world, but retains its value between calls to gimme_another

       If this function is being sourced in from a separate file
       via "require" or "use", then this is probably just fine.
       If it's all in the main program, you'll need to arrange
       for the "my" to be executed early, either by putting the
       whole block above your main program, or more likely,
       placing merely a "BEGIN" code block around it to make sure
       it gets executed before your program starts to run:

           BEGIN {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
               }
           }

       See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod
       about the special triggered code blocks, "BEGIN",
       "UNITCHECK", "CHECK", "INIT" and "END".

       If declared at the outermost scope (the file scope), then
       lexicals work somewhat like C's file statics.  They are
       available to all functions in that same file declared
       below them, but are inaccessible from outside that file.
       This strategy is sometimes used in modules to create
       private variables that the whole module can see.

   Temporary Values via local()
       WARNING: In general, you should be using "my" instead of



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       "local", because it's faster and safer.  Exceptions to
       this include the global punctuation variables, global
       filehandles and formats, and direct manipulation of the
       Perl symbol table itself.  "local" is mostly used when the
       current value of a variable must be visible to called
       subroutines.

       Synopsis:

           # localization of values

           local $foo;                 # make $foo dynamically local
           local (@wid, %get);         # make list of variables local
           local $foo = "flurp";       # make $foo dynamic, and init it
           local @oof = @bar;          # make @oof dynamic, and init it

           local $hash{key} = "val";   # sets a local value for this hash entry
           local ($cond ? $v1 : $v2);  # several types of lvalues support
                                       # localization

           # localization of symbols

           local *FH;                  # localize $FH, @FH, %FH, &FH  ...
           local *merlyn = *randal;    # now $merlyn is really $randal, plus
                                       #     @merlyn is really @randal, etc
           local *merlyn = 'randal';   # SAME THING: promote 'randal' to *randal
           local *merlyn = \$randal;   # just alias $merlyn, not @merlyn etc

       A "local" modifies its listed variables to be "local" to
       the enclosing block, "eval", or "do FILE"--and to any
       subroutine called from within that block.  A "local" just
       gives temporary values to global (meaning package)
       variables.  It does not create a local variable.  This is
       known as dynamic scoping.  Lexical scoping is done with
       "my", which works more like C's auto declarations.

       Some types of lvalues can be localized as well : hash and
       array elements and slices, conditionals (provided that
       their result is always localizable), and symbolic
       references.  As for simple variables, this creates new,
       dynamically scoped values.

       If more than one variable or expression is given to
       "local", they must be placed in parentheses.  This
       operator works by saving the current values of those
       variables in its argument list on a hidden stack and
       restoring them upon exiting the block, subroutine, or
       eval.  This means that called subroutines can also
       reference the local variable, but not the global one.  The
       argument list may be assigned to if desired, which allows
       you to initialize your local variables.  (If no
       initializer is given for a particular variable, it is
       created with an undefined value.)




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       Because "local" is a run-time operator, it gets executed
       each time through a loop.  Consequently, it's more
       efficient to localize your variables outside the loop.

       Grammatical note on local()

       A "local" is simply a modifier on an lvalue expression.
       When you assign to a "local"ized variable, the "local"
       doesn't change whether its list is viewed as a scalar or
       an array.  So

           local($foo) = <STDIN>;
           local @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           local $foo = <STDIN>;

       supplies a scalar context.

       Localization of special variables

       If you localize a special variable, you'll be giving a new
       value to it, but its magic won't go away.  That means that
       all side-effects related to this magic still work with the
       localized value.

       This feature allows code like this to work :

           # Read the whole contents of FILE in $slurp
           { local $/ = undef; $slurp = <FILE>; }

       Note, however, that this restricts localization of some
       values ; for example, the following statement dies, as of
       perl 5.9.0, with an error Modification of a read-only
       value attempted, because the $1 variable is magical and
       read-only :

           local $1 = 2;

       Similarly, but in a way more difficult to spot, the
       following snippet will die in perl 5.9.0 :

           sub f { local $_ = "foo"; print }
           for ($1) {
               # now $_ is aliased to $1, thus is magic and readonly
               f();
           }

       See next section for an alternative to this situation.

       WARNING: Localization of tied arrays and hashes does not
       currently work as described.  This will be fixed in a
       future release of Perl; in the meantime, avoid code that



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       relies on any particular behaviour of localising tied
       arrays or hashes (localising individual elements is still
       okay).  See "Localising Tied Arrays and Hashes Is Broken"
       in perl58delta for more details.

       Localization of globs

       The construct

           local *name;

       creates a whole new symbol table entry for the glob "name"
       in the current package.  That means that all variables in
       its glob slot ($name, @name, %name, &name, and the "name"
       filehandle) are dynamically reset.

       This implies, among other things, that any magic
       eventually carried by those variables is locally lost.  In
       other words, saying "local */" will not have any effect on
       the internal value of the input record separator.

       Notably, if you want to work with a brand new value of the
       default scalar $_, and avoid the potential problem listed
       above about $_ previously carrying a magic value, you
       should use "local *_" instead of "local $_".  As of perl
       5.9.1, you can also use the lexical form of $_ (declaring
       it with "my $_"), which avoids completely this problem.

       Localization of elements of composite types

       It's also worth taking a moment to explain what happens
       when you "local"ize a member of a composite type (i.e. an
       array or hash element).  In this case, the element is
       "local"ized by name. This means that when the scope of the
       "local()" ends, the saved value will be restored to the
       hash element whose key was named in the "local()", or the
       array element whose index was named in the "local()".  If
       that element was deleted while the "local()" was in effect
       (e.g. by a "delete()" from a hash or a "shift()" of an
       array), it will spring back into existence, possibly
       extending an array and filling in the skipped elements
       with "undef".  For instance, if you say

           %hash = ( 'This' => 'is', 'a' => 'test' );
           @ary  = ( 0..5 );
           {
                local($ary[5]) = 6;
                local($hash{'a'}) = 'drill';
                while (my $e = pop(@ary)) {
                    print "$e . . .\n";
                    last unless $e > 3;
                }
                if (@ary) {
                    $hash{'only a'} = 'test';



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                    delete $hash{'a'};
                }
           }
           print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
           print "The array has ",scalar(@ary)," elements: ",
                 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";

       Perl will print

           6 . . .
           4 . . .
           3 . . .
           This is a test only a test.
           The array has 6 elements: 0, 1, 2, undef, undef, 5

       The behavior of local() on non-existent members of
       composite types is subject to change in future.

   Lvalue subroutines
       WARNING: Lvalue subroutines are still experimental and the
       implementation may change in future versions of Perl.

       It is possible to return a modifiable value from a
       subroutine.  To do this, you have to declare the
       subroutine to return an lvalue.

           my $val;
           sub canmod : lvalue {
               # return $val; this doesn't work, don't say "return"
               $val;
           }
           sub nomod {
               $val;
           }

           canmod() = 5;   # assigns to $val
           nomod()  = 5;   # ERROR

       The scalar/list context for the subroutine and for the
       right-hand side of assignment is determined as if the
       subroutine call is replaced by a scalar. For example,
       consider:

           data(2,3) = get_data(3,4);

       Both subroutines here are called in a scalar context,
       while in:

           (data(2,3)) = get_data(3,4);

       and in:

           (data(2),data(3)) = get_data(3,4);




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       all the subroutines are called in a list context.

       Lvalue subroutines are EXPERIMENTAL
           They appear to be convenient, but there are several
           reasons to be circumspect.

           You can't use the return keyword, you must pass out
           the value before falling out of subroutine scope. (see
           comment in example above).  This is usually not a
           problem, but it disallows an explicit return out of a
           deeply nested loop, which is sometimes a nice way out.

           They violate encapsulation.  A normal mutator can
           check the supplied argument before setting the
           attribute it is protecting, an lvalue subroutine never
           gets that chance.  Consider;

               my $some_array_ref = [];    # protected by mutators ??

               sub set_arr {               # normal mutator
                   my $val = shift;
                   die("expected array, you supplied ", ref $val)
                      unless ref $val eq 'ARRAY';
                   $some_array_ref = $val;
               }
               sub set_arr_lv : lvalue {   # lvalue mutator
                   $some_array_ref;
               }

               # set_arr_lv cannot stop this !
               set_arr_lv() = { a => 1 };

   Passing Symbol Table Entries (typeglobs)
       WARNING: The mechanism described in this section was
       originally the only way to simulate pass-by-reference in
       older versions of Perl.  While it still works fine in
       modern versions, the new reference mechanism is generally
       easier to work with.  See below.

       Sometimes you don't want to pass the value of an array to
       a subroutine but rather the name of it, so that the
       subroutine can modify the global copy of it rather than
       working with a local copy.  In perl you can refer to all
       objects of a particular name by prefixing the name with a
       star: *foo.  This is often known as a "typeglob", because
       the star on the front can be thought of as a wildcard
       match for all the funny prefix characters on variables and
       subroutines and such.

       When evaluated, the typeglob produces a scalar value that
       represents all the objects of that name, including any
       filehandle, format, or subroutine.  When assigned to, it
       causes the name mentioned to refer to whatever "*" value
       was assigned to it.  Example:



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           sub doubleary {
               local(*someary) = @_;
               foreach $elem (@someary) {
                   $elem *= 2;
               }
           }
           doubleary(*foo);
           doubleary(*bar);

       Scalars are already passed by reference, so you can modify
       scalar arguments without using this mechanism by referring
       explicitly to $_[0] etc.  You can modify all the elements
       of an array by passing all the elements as scalars, but
       you have to use the "*" mechanism (or the equivalent
       reference mechanism) to "push", "pop", or change the size
       of an array.  It will certainly be faster to pass the
       typeglob (or reference).

       Even if you don't want to modify an array, this mechanism
       is useful for passing multiple arrays in a single LIST,
       because normally the LIST mechanism will merge all the
       array values so that you can't extract out the individual
       arrays.  For more on typeglobs, see "Typeglobs and
       Filehandles" in perldata.

   When to Still Use local()
       Despite the existence of "my", there are still three
       places where the "local" operator still shines.  In fact,
       in these three places, you must use "local" instead of
       "my".

       1.  You need to give a global variable a temporary value,
           especially $_.

           The global variables, like @ARGV or the punctuation
           variables, must be "local"ized with "local()".  This
           block reads in /etc/motd, and splits it up into chunks
           separated by lines of equal signs, which are placed in
           @Fields.

               {
                   local @ARGV = ("/etc/motd");
                   local $/ = undef;
                   local $_ = <>;
                   @Fields = split /^\s*=+\s*$/;
               }

           It particular, it's important to "local"ize $_ in any
           routine that assigns to it.  Look out for implicit
           assignments in "while" conditionals.

       2.  You need to create a local file or directory handle or
           a local function.




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           A function that needs a filehandle of its own must use
           "local()" on a complete typeglob.   This can be used
           to create new symbol table entries:

               sub ioqueue {
                   local  (*READER, *WRITER);    # not my!
                   pipe    (READER,  WRITER)     or die "pipe: $!";
                   return (*READER, *WRITER);
               }
               ($head, $tail) = ioqueue();

           See the Symbol module for a way to create anonymous
           symbol table entries.

           Because assignment of a reference to a typeglob
           creates an alias, this can be used to create what is
           effectively a local function, or at least, a local
           alias.

               {
                   local *grow = \&shrink; # only until this block exists
                   grow();                 # really calls shrink()
                   move();                 # if move() grow()s, it shrink()s too
               }
               grow();                     # get the real grow() again

           See "Function Templates" in perlref for more about
           manipulating functions by name in this way.

       3.  You want to temporarily change just one element of an
           array or hash.

           You can "local"ize just one element of an aggregate.
           Usually this is done on dynamics:

               {
                   local $SIG{INT} = 'IGNORE';
                   funct();                            # uninterruptible
               }
               # interruptibility automatically restored here

           But it also works on lexically declared aggregates.
           Prior to 5.005, this operation could on occasion
           misbehave.

   Pass by Reference
       If you want to pass more than one array or hash into a
       function--or return them from it--and have them maintain
       their integrity, then you're going to have to use an
       explicit pass-by-reference.  Before you do that, you need
       to understand references as detailed in perlref.  This
       section may not make much sense to you otherwise.

       Here are a few simple examples.  First, let's pass in



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       several arrays to a function and have it "pop" all of
       then, returning a new list of all their former last
       elements:

           @tailings = popmany ( \@a, \@b, \@c, \@d );

           sub popmany {
               my $aref;
               my @retlist = ();
               foreach $aref ( @_ ) {
                   push @retlist, pop @$aref;
               }
               return @retlist;
           }

       Here's how you might write a function that returns a list
       of keys occurring in all the hashes passed to it:

           @common = inter( \%foo, \%bar, \%joe );
           sub inter {
               my ($k, $href, %seen); # locals
               foreach $href (@_) {
                   while ( $k = each %$href ) {
                       $seen{$k}++;
                   }
               }
               return grep { $seen{$_} == @_ } keys %seen;
           }

       So far, we're using just the normal list return mechanism.
       What happens if you want to pass or return a hash?  Well,
       if you're using only one of them, or you don't mind them
       concatenating, then the normal calling convention is ok,
       although a little expensive.

       Where people get into trouble is here:

           (@a, @b) = func(@c, @d);
       or
           (%a, %b) = func(%c, %d);

       That syntax simply won't work.  It sets just @a or %a and
       clears the @b or %b.  Plus the function didn't get passed
       into two separate arrays or hashes: it got one long list
       in @_, as always.

       If you can arrange for everyone to deal with this through
       references, it's cleaner code, although not so nice to
       look at.  Here's a function that takes two array
       references as arguments, returning the two array elements
       in order of how many elements they have in them:






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           ($aref, $bref) = func(\@c, \@d);
           print "@$aref has more than @$bref\n";
           sub func {
               my ($cref, $dref) = @_;
               if (@$cref > @$dref) {
                   return ($cref, $dref);
               } else {
                   return ($dref, $cref);
               }
           }

       It turns out that you can actually do this also:

           (*a, *b) = func(\@c, \@d);
           print "@a has more than @b\n";
           sub func {
               local (*c, *d) = @_;
               if (@c > @d) {
                   return (\@c, \@d);
               } else {
                   return (\@d, \@c);
               }
           }

       Here we're using the typeglobs to do symbol table
       aliasing.  It's a tad subtle, though, and also won't work
       if you're using "my" variables, because only globals (even
       in disguise as "local"s) are in the symbol table.

       If you're passing around filehandles, you could usually
       just use the bare typeglob, like *STDOUT, but typeglobs
       references work, too.  For example:

           splutter(\*STDOUT);
           sub splutter {
               my $fh = shift;
               print $fh "her um well a hmmm\n";
           }

           $rec = get_rec(\*STDIN);
           sub get_rec {
               my $fh = shift;
               return scalar <$fh>;
           }

       If you're planning on generating new filehandles, you
       could do this.  Notice to pass back just the bare *FH, not
       its reference.

           sub openit {
               my $path = shift;
               local *FH;
               return open (FH, $path) ? *FH : undef;
           }



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   Prototypes
       Perl supports a very limited kind of compile-time argument
       checking using function prototyping.  If you declare

           sub mypush (\@@)

       then "mypush()" takes arguments exactly like "push()"
       does.  The function declaration must be visible at compile
       time.  The prototype affects only interpretation of new-
       style calls to the function, where new-style is defined as
       not using the "&" character.  In other words, if you call
       it like a built-in function, then it behaves like a built-
       in function.  If you call it like an old-fashioned
       subroutine, then it behaves like an old-fashioned
       subroutine.  It naturally falls out from this rule that
       prototypes have no influence on subroutine references like
       "\&foo" or on indirect subroutine calls like "&{$subref}"
       or "$subref->()".

       Method calls are not influenced by prototypes either,
       because the function to be called is indeterminate at
       compile time, since the exact code called depends on
       inheritance.

       Because the intent of this feature is primarily to let you
       define subroutines that work like built-in functions, here
       are prototypes for some other functions that parse almost
       exactly like the corresponding built-in.

           Declared as                 Called as

           sub mylink ($$)          mylink $old, $new
           sub myvec ($$$)          myvec $var, $offset, 1
           sub myindex ($$;$)       myindex &getstring, "substr"
           sub mysyswrite ($$$;$)   mysyswrite $buf, 0, length($buf) - $off, $off
           sub myreverse (@)        myreverse $a, $b, $c
           sub myjoin ($@)          myjoin ":", $a, $b, $c
           sub mypop (\@)           mypop @array
           sub mysplice (\@$$@)     mysplice @array, 0, 2, @pushme
           sub mykeys (\%)          mykeys %{$hashref}
           sub myopen (*;$)         myopen HANDLE, $name
           sub mypipe (**)          mypipe READHANDLE, WRITEHANDLE
           sub mygrep (&@)          mygrep { /foo/ } $a, $b, $c
           sub myrand (;$)          myrand 42
           sub mytime ()            mytime

       Any backslashed prototype character represents an actual
       argument that absolutely must start with that character.
       The value passed as part of @_ will be a reference to the
       actual argument given in the subroutine call, obtained by
       applying "\" to that argument.

       You can also backslash several argument types
       simultaneously by using the "\[]" notation:



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           sub myref (\[$@%&*])

       will allow calling myref() as

           myref $var
           myref @array
           myref %hash
           myref &sub
           myref *glob

       and the first argument of myref() will be a reference to a
       scalar, an array, a hash, a code, or a glob.

       Unbackslashed prototype characters have special meanings.
       Any unbackslashed "@" or "%" eats all remaining arguments,
       and forces list context.  An argument represented by "$"
       forces scalar context.  An "&" requires an anonymous
       subroutine, which, if passed as the first argument, does
       not require the "sub" keyword or a subsequent comma.

       A "*" allows the subroutine to accept a bareword,
       constant, scalar expression, typeglob, or a reference to a
       typeglob in that slot.  The value will be available to the
       subroutine either as a simple scalar, or (in the latter
       two cases) as a reference to the typeglob.  If you wish to
       always convert such arguments to a typeglob reference, use
       Symbol::qualify_to_ref() as follows:

           use Symbol 'qualify_to_ref';

           sub foo (*) {
               my $fh = qualify_to_ref(shift, caller);
               ...
           }

       A semicolon (";") separates mandatory arguments from
       optional arguments.  It is redundant before "@" or "%",
       which gobble up everything else.

       As the last character of a prototype, or just before a
       semicolon, you can use "_" in place of "$": if this
       argument is not provided, $_ will be used instead.

       Note how the last three examples in the table above are
       treated specially by the parser.  "mygrep()" is parsed as
       a true list operator, "myrand()" is parsed as a true unary
       operator with unary precedence the same as "rand()", and
       "mytime()" is truly without arguments, just like "time()".
       That is, if you say

           mytime +2;

       you'll get "mytime() + 2", not mytime(2), which is how it
       would be parsed without a prototype.



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       The interesting thing about "&" is that you can generate
       new syntax with it, provided it's in the initial position:

           sub try (&@) {
               my($try,$catch) = @_;
               eval { &$try };
               if ($@) {
                   local $_ = $@;
                   &$catch;
               }
           }
           sub catch (&) { $_[0] }

           try {
               die "phooey";
           } catch {
               /phooey/ and print "unphooey\n";
           };

       That prints "unphooey".  (Yes, there are still unresolved
       issues having to do with visibility of @_.  I'm ignoring
       that question for the moment.  (But note that if we make
       @_ lexically scoped, those anonymous subroutines can act
       like closures... (Gee, is this sounding a little Lispish?
       (Never mind.))))

       And here's a reimplementation of the Perl "grep" operator:

           sub mygrep (&@) {
               my $code = shift;
               my @result;
               foreach $_ (@_) {
                   push(@result, $_) if &$code;
               }
               @result;
           }

       Some folks would prefer full alphanumeric prototypes.
       Alphanumerics have been intentionally left out of
       prototypes for the express purpose of someday in the
       future adding named, formal parameters.  The current
       mechanism's main goal is to let module writers provide
       better diagnostics for module users.  Larry feels the
       notation quite understandable to Perl programmers, and
       that it will not intrude greatly upon the meat of the
       module, nor make it harder to read.  The line noise is
       visually encapsulated into a small pill that's easy to
       swallow.

       If you try to use an alphanumeric sequence in a prototype
       you will generate an optional warning - "Illegal character
       in prototype...".  Unfortunately earlier versions of Perl
       allowed the prototype to be used as long as its prefix was
       a valid prototype.  The warning may be upgraded to a fatal



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       error in a future version of Perl once the majority of
       offending code is fixed.

       It's probably best to prototype new functions, not
       retrofit prototyping into older ones.  That's because you
       must be especially careful about silent impositions of
       differing list versus scalar contexts.  For example, if
       you decide that a function should take just one parameter,
       like this:

           sub func ($) {
               my $n = shift;
               print "you gave me $n\n";
           }

       and someone has been calling it with an array or
       expression returning a list:

           func(@foo);
           func( split /:/ );

       Then you've just supplied an automatic "scalar" in front
       of their argument, which can be more than a bit
       surprising.  The old @foo which used to hold one thing
       doesn't get passed in.  Instead, "func()" now gets passed
       in a 1; that is, the number of elements in @foo.  And the
       "split" gets called in scalar context so it starts
       scribbling on your @_ parameter list.  Ouch!

       This is all very powerful, of course, and should be used
       only in moderation to make the world a better place.

   Constant Functions
       Functions with a prototype of "()" are potential
       candidates for inlining.  If the result after optimization
       and constant folding is either a constant or a lexically-
       scoped scalar which has no other references, then it will
       be used in place of function calls made without "&".
       Calls made using "&" are never inlined.  (See constant.pm
       for an easy way to declare most constants.)

       The following functions would all be inlined:

           sub pi ()           { 3.14159 }             # Not exact, but close.
           sub PI ()           { 4 * atan2 1, 1 }      # As good as it gets,
                                                       # and it's inlined, too!
           sub ST_DEV ()       { 0 }
           sub ST_INO ()       { 1 }

           sub FLAG_FOO ()     { 1 << 8 }
           sub FLAG_BAR ()     { 1 << 9 }
           sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

           sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }



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           sub N () { int(OPT_BAZ) / 3 }

           sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }

       Be aware that these will not be inlined; as they contain
       inner scopes, the constant folding doesn't reduce them to
       a single constant:

           sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }

           sub baz_val () {
               if (OPT_BAZ) {
                   return 23;
               }
               else {
                   return 42;
               }
           }

       If you redefine a subroutine that was eligible for
       inlining, you'll get a mandatory warning.  (You can use
       this warning to tell whether or not a particular
       subroutine is considered constant.)  The warning is
       considered severe enough not to be optional because
       previously compiled invocations of the function will still
       be using the old value of the function.  If you need to be
       able to redefine the subroutine, you need to ensure that
       it isn't inlined, either by dropping the "()" prototype
       (which changes calling semantics, so beware) or by
       thwarting the inlining mechanism in some other way, such
       as

           sub not_inlined () {
               23 if $];
           }

   Overriding Built-in Functions
       Many built-in functions may be overridden, though this
       should be tried only occasionally and for good reason.
       Typically this might be done by a package attempting to
       emulate missing built-in functionality on a non-Unix
       system.

       Overriding may be done only by importing the name from a
       module at compile time--ordinary predeclaration isn't good
       enough.  However, the "use subs" pragma lets you, in
       effect, predeclare subs via the import syntax, and these
       names may then override built-in ones:

           use subs 'chdir', 'chroot', 'chmod', 'chown';
           chdir $somewhere;
           sub chdir { ... }




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       To unambiguously refer to the built-in form, precede the
       built-in name with the special package qualifier "CORE::".
       For example, saying "CORE::open()" always refers to the
       built-in "open()", even if the current package has
       imported some other subroutine called "&open()" from
       elsewhere.  Even though it looks like a regular function
       call, it isn't: you can't take a reference to it, such as
       the incorrect "\&CORE::open" might appear to produce.

       Library modules should not in general export built-in
       names like "open" or "chdir" as part of their default
       @EXPORT list, because these may sneak into someone else's
       namespace and change the semantics unexpectedly.  Instead,
       if the module adds that name to @EXPORT_OK, then it's
       possible for a user to import the name explicitly, but not
       implicitly.  That is, they could say

           use Module 'open';

       and it would import the "open" override.  But if they said

           use Module;

       they would get the default imports without overrides.

       The foregoing mechanism for overriding built-in is
       restricted, quite deliberately, to the package that
       requests the import.  There is a second method that is
       sometimes applicable when you wish to override a built-in
       everywhere, without regard to namespace boundaries.  This
       is achieved by importing a sub into the special namespace
       "CORE::GLOBAL::".  Here is an example that quite brazenly
       replaces the "glob" operator with something that
       understands regular expressions.

           package REGlob;
           require Exporter;
           @ISA = 'Exporter';
           @EXPORT_OK = 'glob';

           sub import {
               my $pkg = shift;
               return unless @_;
               my $sym = shift;
               my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
               $pkg->export($where, $sym, @_);
           }

           sub glob {
               my $pat = shift;
               my @got;
               if (opendir my $d, '.') {
                   @got = grep /$pat/, readdir $d;
                   closedir $d;



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               }
               return @got;
           }
           1;

       And here's how it could be (ab)used:

           #use REGlob 'GLOBAL_glob';      # override glob() in ALL namespaces
           package Foo;
           use REGlob 'glob';              # override glob() in Foo:: only
           print for <^[a-z_]+\.pm\$>;     # show all pragmatic modules

       The initial comment shows a contrived, even dangerous
       example.  By overriding "glob" globally, you would be
       forcing the new (and subversive) behavior for the "glob"
       operator for every namespace, without the complete
       cognizance or cooperation of the modules that own those
       namespaces.  Naturally, this should be done with extreme
       caution--if it must be done at all.

       The "REGlob" example above does not implement all the
       support needed to cleanly override perl's "glob" operator.
       The built-in "glob" has different behaviors depending on
       whether it appears in a scalar or list context, but our
       "REGlob" doesn't.  Indeed, many perl built-in have such
       context sensitive behaviors, and these must be adequately
       supported by a properly written override.  For a fully
       functional example of overriding "glob", study the
       implementation of "File::DosGlob" in the standard library.

       When you override a built-in, your replacement should be
       consistent (if possible) with the built-in native syntax.
       You can achieve this by using a suitable prototype.  To
       get the prototype of an overridable built-in, use the
       "prototype" function with an argument of
       "CORE::builtin_name" (see "prototype" in perlfunc).

       Note however that some built-ins can't have their syntax
       expressed by a prototype (such as "system" or "chomp").
       If you override them you won't be able to fully mimic
       their original syntax.

       The built-ins "do", "require" and "glob" can also be
       overridden, but due to special magic, their original
       syntax is preserved, and you don't have to define a
       prototype for their replacements.  (You can't override the
       "do BLOCK" syntax, though).

       "require" has special additional dark magic: if you invoke
       your "require" replacement as "require Foo::Bar", it will
       actually receive the argument "Foo/Bar.pm" in @_.  See
       "require" in perlfunc.

       And, as you'll have noticed from the previous example, if



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       you override "glob", the "<*>" glob operator is overridden
       as well.

       In a similar fashion, overriding the "readline" function
       also overrides the equivalent I/O operator "<FILEHANDLE>".
       Also, overriding "readpipe" also overrides the operators
       "``" and "qx//".

       Finally, some built-ins (e.g. "exists" or "grep") can't be
       overridden.

   Autoloading
       If you call a subroutine that is undefined, you would
       ordinarily get an immediate, fatal error complaining that
       the subroutine doesn't exist.  (Likewise for subroutines
       being used as methods, when the method doesn't exist in
       any base class of the class's package.)  However, if an
       "AUTOLOAD" subroutine is defined in the package or
       packages used to locate the original subroutine, then that
       "AUTOLOAD" subroutine is called with the arguments that
       would have been passed to the original subroutine.  The
       fully qualified name of the original subroutine magically
       appears in the global $AUTOLOAD variable of the same
       package as the "AUTOLOAD" routine.  The name is not passed
       as an ordinary argument because, er, well, just because,
       that's why.  (As an exception, a method call to a
       nonexistent "import" or "unimport" method is just skipped
       instead.)

       Many "AUTOLOAD" routines load in a definition for the
       requested subroutine using eval(), then execute that
       subroutine using a special form of goto() that erases the
       stack frame of the "AUTOLOAD" routine without a trace.
       (See the source to the standard module documented in
       AutoLoader, for example.)  But an "AUTOLOAD" routine can
       also just emulate the routine and never define it.   For
       example, let's pretend that a function that wasn't defined
       should just invoke "system" with those arguments.  All
       you'd do is:

           sub AUTOLOAD {
               my $program = $AUTOLOAD;
               $program =~ s/.*:://;
               system($program, @_);
           }
           date();
           who('am', 'i');
           ls('-l');

       In fact, if you predeclare functions you want to call that
       way, you don't even need parentheses:






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           use subs qw(date who ls);
           date;
           who "am", "i";
           ls '-l';

       A more complete example of this is the standard Shell
       module, which can treat undefined subroutine calls as
       calls to external programs.

       Mechanisms are available to help modules writers split
       their modules into autoloadable files.  See the standard
       AutoLoader module described in AutoLoader and in
       AutoSplit, the standard SelfLoader modules in SelfLoader,
       and the document on adding C functions to Perl code in
       perlxs.

   Subroutine Attributes
       A subroutine declaration or definition may have a list of
       attributes associated with it.  If such an attribute list
       is present, it is broken up at space or colon boundaries
       and treated as though a "use attributes" had been seen.
       See attributes for details about what attributes are
       currently supported.  Unlike the limitation with the
       obsolescent "use attrs", the "sub : ATTRLIST" syntax works
       to associate the attributes with a pre-declaration, and
       not just with a subroutine definition.

       The attributes must be valid as simple identifier names
       (without any punctuation other than the '_' character).
       They may have a parameter list appended, which is only
       checked for whether its parentheses ('(',')') nest
       properly.

       Examples of valid syntax (even though the attributes are
       unknown):

           sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
           sub plugh () : Ugly('\(") :Bad;
           sub xyzzy : _5x5 { ... }

       Examples of invalid syntax:

           sub fnord : switch(10,foo(); # ()-string not balanced
           sub snoid : Ugly('(');        # ()-string not balanced
           sub xyzzy : 5x5;              # "5x5" not a valid identifier
           sub plugh : Y2::north;        # "Y2::north" not a simple identifier
           sub snurt : foo + bar;        # "+" not a colon or space

       The attribute list is passed as a list of constant strings
       to the code which associates them with the subroutine.  In
       particular, the second example of valid syntax above
       currently looks like this in terms of how it's parsed and
       invoked:




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           use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';

       For further details on attribute lists and their
       manipulation, see attributes and Attribute::Handlers.

SEE ALSO
       See "Function Templates" in perlref for more about
       references and closures.  See perlxs if you'd like to
       learn about calling C subroutines from Perl.  See
       perlembed if you'd like to learn about calling Perl
       subroutines from C.  See perlmod to learn about bundling
       up your functions in separate files.  See perlmodlib to
       learn what library modules come standard on your system.
       See perltoot to learn how to make object method calls.











































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