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

NNAAMMEE
       perlsub - Perl subroutines

SSYYNNOOPPSSIISS
       To declare subroutines:

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

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

       To define an anonymous subroutine at runtime:

           $subref = sub BLOCK;            # no proto
           $subref = sub (PROTO) BLOCK;    # with proto

       To import subroutines:

           use PACKAGE qw(NAME1 NAME2 NAME3);

       To call subroutines:

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

DDEESSCCRRIIPPTTIIOONN
       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 even generated on the fly using eval
       or anonymous subroutines (closures).  You can even call a
       function indirectly using a variable containing its name
       or a CODE reference to it.

       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 the language's perspective.)

       Any arguments passed to the routine come in as the array
       @_.  Thus 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 if a reference to it is taken.
       (Some earlier versions of Perl created the element whether
       or not it was assigned to.)  Note that assigning to the
       whole array @_ removes the aliasing, and does not update
       any arguments.

       The return value of the subroutine is the value of the
       last expression evaluated.  Alternatively, a return
       statement may be used to exit the 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 will return an empty list in
       a list context, an undefined value in a scalar context, or
       nothing in a void context.  If you return one or more
       arrays and/or hashes, these will be flattened together
       into one large indistinguishable list.

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

       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;
                   }
               }
               $thisline;
           }

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

       Use array assignment to a local list to name your formal
       arguments:

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

       This also has the effect of turning call-by-reference into
       call-by-value, because the assignment copies the values.
       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:

           ($v3, $v4) = upcase($v1, $v2);  # this doesn't
           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 will
       see everything as one big long flat @_ parameter list.
       This is one of the ways 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);

       Because like its flat incoming parameter list, the return
       list is also flat.  So all you have managed to do here is
       stored everything in @a and made @b an empty list.  See
       the section on Pass by Reference for alternatives.

       A subroutine may be called using the "&" prefix.  The "&"
       is optional in modern Perls, and so are the parentheses if
       the subroutine has been predeclared.  (Note, however, that
       the "&" is NOT optional when you're 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.
       See the perlref manpage for more on 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"

       Not only does the "&" form make the argument list
       optional, but it also disables any prototype checking on
       the 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 the section
       on Prototypes below.

       Function whose names are in all upper case are reserved to
       the Perl core, just as are modules whose names are in all
       lower case.  A function in all capitals is a loosely-held
       convention meaning it will be called indirectly by the
       run-time system itself.  Functions that do special, pre-
       defined things are BEGIN, END, AUTOLOAD, and DESTROY--plus
       all the functions mentioned in the perltie manpage.  The
       5.005 release adds INIT to this list.

       PPrriivvaattee VVaarriiaabblleess vviiaa 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

       A "my" declares the listed variables to be confined
       (lexically) 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 builtins like
       $/ must currently be localize 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
       (even 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 the 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 the file scope.

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

       (An eval(), however, can see the 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 the
       perlref manpage.)

       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 the 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 the variables in its
       argument list, the "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 the 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 the scope, too.
       Thus in the loop

           while (defined(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
       throughout the rest of the conditional (including elsif
       and else clauses, if any), but not beyond it.

       (None of the foregoing applies to if/unless or while/until
       modifiers appended to simple statements.  Such modifiers
       are not control structures and have no effect on scoping.)

       The foreach loop defaults to scoping its index variable
       dynamically (in the manner of local; see below).  However,
       if the index variable is prefixed with the keyword "my",
       then it is lexically scoped 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, and so the value of $i is unavailable in
       some_function().

       Some users may wish to encourage the use of lexically
       scoped variables.  As an aid to catching implicit
       references to package variables, if you say

           use strict 'vars';

       then any variable reference from there to the end of the
       enclosing block must either refer to a lexical variable,
       or 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
       principle usefulness of this is to quiet "use strict
       'vars'".  The actual initialization is delayed until run
       time, so it gets executed appropriately; every 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
           my $_;              # also illegal (currently)

       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 totally from the outside
       world.  This is similar to C's static variables at the
       file level.  To do this with a subroutine requires the use
       of a closure (anonymous function with lexical access).  If
       a block (such as an eval(), function, or package) wants 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();

       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.

       PPeerrssiisstteenntt PPrriivvaattee VVaarriiaabblleess

       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 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 sub around it to make sure it gets executed
       before your program starts to run:

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

       See the section on Package Constructors and Destructors in
       the perlmod manpage about the BEGIN function.

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

       TTeemmppoorraarryy VVaalluueess vviiaa local()

       NNOOTTEE: In general, you should be using "my" instead of
       "local", because it's faster and safer.  Exceptions to
       this include the global punctuation variables, filehandles
       and formats, and direct manipulation of the Perl symbol
       table itself.  Format variables often use "local" though,
       as do other variables whose current value must be visible
       to called subroutines.

       Synopsis:

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

           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 nnoott create a local variable.  This is
       known as dynamic scoping.  Lexical scoping is done with
       "my", which works more like C's auto declarations.

       If more than one variable is given to local(), they must
       be placed in parentheses.  All listed elements must be
       legal lvalues.  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.)  Commonly this is used
       to name the parameters to a subroutine.  Examples:

           for $i ( 0 .. 9 ) {
               $digits{$i} = $i;
           }
           # assume this function uses global %digits hash
           parse_num();

           # now temporarily add to %digits hash
           if ($base12) {
               # (NOTE: not claiming this is efficient!)
               local %digits  = (%digits, 't' => 10, 'e' => 11);
               parse_num();  # parse_num gets this new %digits!
           }
           # old %digits restored here

       Because local() is a run-time command, it gets executed
       every time through a loop.  In releases of Perl previous
       to 5.0, this used more stack storage each time until the
       loop was exited.  Perl now reclaims the space each time
       through, but it's still more efficient to declare your
       variables outside the loop.

       A local is simply a modifier on an lvalue expression.
       When you assign to a localized 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.

       A note about local() and composite types is in order.
       Something like local(%foo) works by temporarily placing a
       brand new hash in the symbol table.  The old hash is left
       alone, but is hidden "behind" the new one.

       This means the old variable is completely invisible via
       the symbol table (i.e. the hash entry in the *foo
       typeglob) for the duration of the dynamic scope within
       which the local() was seen.  This has the effect of
       allowing one to temporarily occlude any magic on composite
       types.  For instance, this will briefly alter a tied hash
       to some other implementation:

           tie %ahash, 'APackage';
           [...]
           {
              local %ahash;
              tie %ahash, 'BPackage';
              [..called code will see %ahash tied to 'BPackage'..]
              {
                 local %ahash;
                 [..%ahash is a normal (untied) hash here..]
              }
           }
           [..%ahash back to its initial tied self again..]

       As another example, a custom implementation of %ENV might
       look like this:

           {
               local %ENV;
               tie %ENV, 'MyOwnEnv';
               [..do your own fancy %ENV manipulation here..]
           }
           [..normal %ENV behavior here..]

       It's also worth taking a moment to explain what happens
       when you localize a member of a composite type (i.e. an
       array or hash element).  In this case, the element is
       localized 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

       Note also that when you localize a member of a composite
       type that ddooeess nnoott eexxiisstt pprreevviioouussllyy, the value is treated
       as though it were in an lvalue context, i.e., it is first
       created and then localized.  The consequence of this is
       that the hash or array is in fact permanently modified.
       For instance, if you say

           %hash = ( 'This' => 'is', 'a' => 'test' );
           @ary  = ( 0..5 );
           {
               local($ary[8]) = 0;
               local($hash{'b'}) = 'whatever';
           }
           printf "%%hash has now %d keys, \@ary %d elements.\n",
               scalar(keys(%hash)), scalar(@ary);

       Perl will print

           %hash has now 3 keys, @ary 9 elements.

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

       PPaassssiinngg SSyymmbbooll TTaabbllee EEnnttrriieess ((ttyyppeegglloobbss))

       [Note:  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;
               }
           }
           doubleary(*foo);
           doubleary(*bar);

       Note that 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 the section on
       Typeglobs and Filehandles in the perldata manpage.

       WWhheenn ttoo SSttiillll UUssee 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 localized 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 localize $_ 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() uses local() on 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 the section on Function Templates in the perlref
            manpage 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 localize 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.

       PPaassss bbyy RReeffeerreennccee

       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 the perlref
       manpage.  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,
       return 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:

           ($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
       (well, and 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 would be better because they'll still work
       properly under use strict 'refs'.  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>;
           }

       Another way to do this is using *HANDLE{IO}, see the
       perlref manpage for usage and caveats.

       If you're planning on generating new filehandles, you
       could do this:

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

       Although that will actually produce a small memory leak.
       See the bottom of the open() entry in the perlfunc manpage
       for a somewhat cleaner way using the IO::Handle package.

       PPrroottoottyyppeess

       As of the 5.002 release of perl, if you declare

           sub mypush (\@@)

       then mypush() takes arguments exactly like push() does.
       The declaration of the function to be called must be
       visible at compile time.  The prototype affects only the
       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 builtin function, then
       it behaves like a builtin 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, because it depends on inheritance.

       Because the intent is primarily to let you define
       subroutines that work like builtin commands, here are the
       prototypes for some other functions that parse almost
       exactly like the corresponding builtins.

           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, @array, 0, @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 to the subroutine (as part of @_) will be
       a reference to the actual argument given in the subroutine
       call, obtained by applying \ to that argument.

       Unbackslashed prototype characters have special meanings.
       Any unbackslashed @ or % eats all the rest of the
       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.

       A semicolon separates mandatory arguments from optional
       arguments.  (It is redundant before @ or %.)

       Note how the last three examples 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 the prototype.

       The interesting thing about & is that you can generate new
       syntax with it:

           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 the 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 grep:

           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.

       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, the func() now gets
       passed in 1, that is, the number of elements in @foo.  And
       the split() gets called in a scalar context and starts
       scribbling on your @_ parameter list.

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

       CCoonnssttaanntt FFuunnccttiioonnss

       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 & or do.
       Calls made using & or do 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) }
           sub BAZ_VAL () {
               if (OPT_BAZ) {
                   return 23;
               }
               else {
                   return 42;
               }
           }

           sub N () { int(BAZ_VAL) / 3 }
           BEGIN {
               my $prod = 1;
               for (1..N) { $prod *= $_ }
               sub N_FACTORIAL () { $prod }
           }

       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 the calling semantics, so beware) or by thwarting
       the inlining mechanism in some other way, such as

           sub not_inlined () {
               23 if $];
           }

       OOvveerrrriiddiinngg BBuuiillttiinn FFuunnccttiioonnss

       Many builtin 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 builtin functionality on a non-Unix
       system.

       Overriding may be done only by importing the name from a
       module--ordinary predeclaration isn't good enough.
       However, the subs pragma (compiler directive) lets you, in
       effect, predeclare subs via the import syntax, and these
       names may then override the builtin ones:

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

       To unambiguously refer to the builtin form, one may
       precede the builtin name with the special package
       qualifier CORE::.  For example, saying CORE::open() will
       always refer to the builtin open(), even if the current
       package has imported some other subroutine called &open()
       from elsewhere.

       Library modules should not in general export builtin 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 the name to the @EXPORT_OK list, 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 the overrides.

       The foregoing mechanism for overriding builtins 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 builtin
       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 $pat = shift;
               my @got;
               local(*D);
               if (opendir D, '.') { @got = grep /$pat/, readdir D; closedir D; }
               @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

       Note that 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 eevveerryy 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 builtin glob has different behaviors depending on
       whether it appears in a scalar or list context, but our
       REGlob doesn't.  Indeed, many perl builtins 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.

       AAuuttoollooaaddiinngg

       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 package.) If, however, there
       is an AUTOLOAD subroutine defined in the package or
       packages that were searched for 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 $AUTOLOAD variable in the same
       package as the AUTOLOAD routine.  The name is not passed
       as an ordinary argument because, er, well, just because,
       that's why...

       Most AUTOLOAD routines will load in a definition for the
       subroutine in question using eval, and then execute that
       subroutine using a special form of "goto" that erases the
       stack frame of the AUTOLOAD routine without a trace.  (See
       the standard AutoLoader module, 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 call system()
       with those arguments.  All you'd do is this:

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

       In fact, if you predeclare the functions you want to call
       that way, you don't even need the 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 Unix programs.

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

SSEEEE AALLSSOO
       See the perlref manpage for more about references and
       closures.  See the perlxs manpage if you'd like to learn
       about calling C subroutines from perl.  See the perlmod
       manpage to learn about bundling up your functions in
       separate files.

27/Mar/1999            perl 5.005, patch 03                     1