perlsub


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.)

       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
       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:

           # 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 {

       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:

           ($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

       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.  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.

               $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,

           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) {

       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 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

       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 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
       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 "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
           delete local $hash{key};    # delete this entry for the current block
           local ($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
       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.)

       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;

       One exception is the default scalar variable: starting with perl 5.14
       "local($_)" will always strip all magic from $_, to make it possible to
       safely reuse $_ in a subroutine.

       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 relies on any particular behaviour of
       localising tied arrays or hashes (localising individual elements is
       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.

       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';
                    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.

       Localized deletion of elements of composite types

       You can use the "delete local $array[$idx]" and "delete local
       $hash{key}" constructs to delete a composite type entry for the current
       block and restore it when it ends. They return the array/hash value
           do {
               my $val = $hash{key};
               local  $hash{key};
               delete $hash{key};
               $val
           }

       except that for those the "local" is scoped to the "do" block. Slices
       are also accepted.

           my %hash = (
            a => [ 7, 8, 9 ],
            b => 1,
           )

           {
            my $a = delete local $hash{a};
            # $a is [ 7, 8, 9 ]
            # %hash is (b => 1)

            {
             my @nums = delete local @$a[0, 2]
             # @nums is (7, 9)
             # $a is [ undef, 8 ]

             $a[0] = 999; # will be erased when the scope ends
            }
            # $a is back to [ 7, 8, 9 ]

           }
           # %hash is back to its original state

   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:

       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:

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

       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.

           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()

               {
                   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 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);
           ($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;
           }

       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
       must start with that character (optionally preceded by "my", "our" or
       "local"), with the exception of "$", which will accept a hash or array
       element even without a dollar sign, such as "my_function()->[0]". 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 use the "\[]" backslash group notation to specify more than one
       allowed argument type. For example:

           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.
       Symbol::qualify_to_ref() as follows:

           use Symbol 'qualify_to_ref';

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

       The "+" prototype is a special alternative to "$" that will act like
       "\[@%]" when given a literal array or hash variable, but will otherwise
       force scalar context on the argument.  This is useful for functions
       which should accept either a literal array or an array reference as the
       argument:

           sub mypush (+@) {
               my $aref = shift;
               die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
               push @$aref, @_;
           }

       When using the "+" prototype, your function must check that the
       argument is of an acceptable type.

       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.

       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;
               }
           }

       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 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!


           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) }

           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
       "\&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;
               }
               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

       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 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.  Also, if the AUTOLOAD subroutine is an XSUB,
       $AUTOLOAD is not populated; instead, you should call "SvPVX"/"SvCUR" on
       the "CV" for "AUTOLOAD" to retrieve the method name.)

       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
           who('am', 'i');
           ls('-l');

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

           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:

       how to make object method calls.



perl v5.14.2                      2011-09-26                        PERLSUB(1)
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