1PERLSUB(1) Perl Programmers Reference Guide PERLSUB(1)
2
6 perlsub - Perl subroutines
7
9 To declare subroutines:
10 sub NAME; # A "forward" declaration.
12 sub NAME(PROTO); # ditto, but with prototypes
13 sub NAME : ATTRS; # with attributes
14 sub NAME(PROTO) : ATTRS; # with attributes and prototypes
15 sub NAME BLOCK # A declaration and a definition.
17 sub NAME(PROTO) BLOCK # ditto, but with prototypes
18 sub NAME : ATTRS BLOCK # with attributes
19 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
20 To define an anonymous subroutine at runtime:
22 $subref = sub BLOCK; # no proto
24 $subref = sub (PROTO) BLOCK; # with proto
25 $subref = sub : ATTRS BLOCK; # with attributes
26 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
27 To import subroutines:
29 use MODULE qw(NAME1 NAME2 NAME3);
31 To call subroutines:
33 NAME(LIST); # & is optional with parentheses.
35 NAME LIST; # Parentheses optional if predeclared/imported.
36 &NAME(LIST); # Circumvent prototypes.
37 &NAME; # Makes current @_ visible to called subroutine.
38
40 Like many languages, Perl provides for user-defined subroutines. These
41 may be located anywhere in the main program, loaded in from other files
42 via the "do", "require", or "use" keywords, or generated on the fly
43 using "eval" or anonymous subroutines. You can even call a function
44 indirectly using a variable containing its name or a CODE reference.
45 The Perl model for function call and return values is simple: all
47 functions are passed as parameters one single flat list of scalars, and
48 all functions likewise return to their caller one single flat list of
49 scalars. Any arrays or hashes in these call and return lists will
50 collapse, losing their identities--but you may always use pass-by-
51 reference instead to avoid this. Both call and return lists may
52 contain as many or as few scalar elements as you'd like. (Often a
53 function without an explicit return statement is called a subroutine,
54 but there's really no difference from Perl's perspective.)
55 Any arguments passed in show up in the array @_. Therefore, if you
57 called a function with two arguments, those would be stored in $_[0]
58 and $_[1]. The array @_ is a local array, but its elements are aliases
59 for the actual scalar parameters. In particular, if an element $_[0]
60 is updated, the corresponding argument is updated (or an error occurs
61 if it is not updatable). If an argument is an array or hash element
62 which did not exist when the function was called, that element is
63 created only when (and if) it is modified or a reference to it is
64 taken. (Some earlier versions of Perl created the element whether or
65 not the element was assigned to.) Assigning to the whole array @_
66 removes that aliasing, and does not update any arguments.
67 A "return" statement may be used to exit a subroutine, optionally
69 specifying the returned value, which will be evaluated in the
70 appropriate context (list, scalar, or void) depending on the context of
71 the subroutine call. If you specify no return value, the subroutine
72 returns an empty list in list context, the undefined value in scalar
73 context, or nothing in void context. If you return one or more
74 aggregates (arrays and hashes), these will be flattened together into
75 one large indistinguishable list.
76 If no "return" is found and if the last statement is an expression, its
78 value is returned. If the last statement is a loop control structure
79 like a "foreach" or a "while", the returned value is unspecified. The
80 empty sub returns the empty list.
81 Perl does not have named formal parameters. In practice all you do is
83 assign to a "my()" list of these. Variables that aren't declared to be
84 private are global variables. For gory details on creating private
85 variables, see "Private Variables via my()" and "Temporary Values via
86 local()". To create protected environments for a set of functions in a
87 separate package (and probably a separate file), see "Packages" in
88 perlmod.
89 Example:
91 sub max {
93 my $max = shift(@_);
94 foreach $foo (@_) {
95 $max = $foo if $max < $foo;
96 }
97 return $max;
98 }
99 $bestday = max($mon,$tue,$wed,$thu,$fri);
100 Example:
102 # get a line, combining continuation lines
104 # that start with whitespace
105 sub get_line {
107 $thisline = $lookahead; # global variables!
108 LINE: while (defined($lookahead = <STDIN>)) {
109 if ($lookahead =~ /^[ \t]/) {
110 $thisline .= $lookahead;
111 }
112 else {
113 last LINE;
114 }
115 }
116 return $thisline;
117 }
118 $lookahead = <STDIN>; # get first line
120 while (defined($line = get_line())) {
121 ...
122 }
123 Assigning to a list of private variables to name your arguments:
125 sub maybeset {
127 my($key, $value) = @_;
128 $Foo{$key} = $value unless $Foo{$key};
129 }
130 Because the assignment copies the values, this also has the effect of
132 turning call-by-reference into call-by-value. Otherwise a function is
133 free to do in-place modifications of @_ and change its caller's values.
134 upcase_in($v1, $v2); # this changes $v1 and $v2
136 sub upcase_in {
137 for (@_) { tr/a-z/A-Z/ }
138 }
139 You aren't allowed to modify constants in this way, of course. If an
141 argument were actually literal and you tried to change it, you'd take a
142 (presumably fatal) exception. For example, this won't work:
143 upcase_in("frederick");
145 It would be much safer if the "upcase_in()" function were written to
147 return a copy of its parameters instead of changing them in place:
148 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
150 sub upcase {
151 return unless defined wantarray; # void context, do nothing
152 my @parms = @_;
153 for (@parms) { tr/a-z/A-Z/ }
154 return wantarray ? @parms : $parms[0];
155 }
156 Notice how this (unprototyped) function doesn't care whether it was
158 passed real scalars or arrays. Perl sees all arguments as one big,
159 long, flat parameter list in @_. This is one area where Perl's simple
160 argument-passing style shines. The "upcase()" function would work
161 perfectly well without changing the "upcase()" definition even if we
162 fed it things like this:
163 @newlist = upcase(@list1, @list2);
165 @newlist = upcase( split /:/, $var );
166 Do not, however, be tempted to do this:
168 (@a, @b) = upcase(@list1, @list2);
170 Like the flattened incoming parameter list, the return list is also
172 flattened on return. So all you have managed to do here is stored
173 everything in @a and made @b empty. See "Pass by Reference" for
174 alternatives.
175 A subroutine may be called using an explicit "&" prefix. The "&" is
177 optional in modern Perl, as are parentheses if the subroutine has been
178 predeclared. The "&" is not optional when just naming the subroutine,
179 such as when it's used as an argument to defined() or undef(). Nor is
180 it optional when you want to do an indirect subroutine call with a
181 subroutine name or reference using the "&$subref()" or "&{$subref}()"
182 constructs, although the "$subref->()" notation solves that problem.
183 See perlref for more about all that.
184 Subroutines may be called recursively. If a subroutine is called using
186 the "&" form, the argument list is optional, and if omitted, no @_
187 array is set up for the subroutine: the @_ array at the time of the
188 call is visible to subroutine instead. This is an efficiency mechanism
189 that new users may wish to avoid.
190 &foo(1,2,3); # pass three arguments
192 foo(1,2,3); # the same
193 foo(); # pass a null list
195 &foo(); # the same
196 &foo; # foo() get current args, like foo(@_) !!
198 foo; # like foo() IFF sub foo predeclared, else "foo"
199 Not only does the "&" form make the argument list optional, it also
201 disables any prototype checking on arguments you do provide. This is
202 partly for historical reasons, and partly for having a convenient way
203 to cheat if you know what you're doing. See Prototypes below.
204 Subroutines whose names are in all upper case are reserved to the Perl
206 core, as are modules whose names are in all lower case. A subroutine
207 in all capitals is a loosely-held convention meaning it will be called
208 indirectly by the run-time system itself, usually due to a triggered
209 event. Subroutines that do special, pre-defined things include
210 "AUTOLOAD", "CLONE", "DESTROY" plus all functions mentioned in perltie
211 and PerlIO::via.
212 The "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END" subroutines are not
214 so much subroutines as named special code blocks, of which you can have
215 more than one in a package, and which you can not call explicitly. See
216 "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod
217 Private Variables via my()
219 Synopsis:
220 my $foo; # declare $foo lexically local
222 my (@wid, %get); # declare list of variables local
223 my $foo = "flurp"; # declare $foo lexical, and init it
224 my @oof = @bar; # declare @oof lexical, and init it
225 my $x : Foo = $y; # similar, with an attribute applied
226 WARNING: The use of attribute lists on "my" declarations is still
228 evolving. The current semantics and interface are subject to change.
229 See attributes and Attribute::Handlers.
230 The "my" operator declares the listed variables to be lexically
232 confined to the enclosing block, conditional ("if/unless/elsif/else"),
233 loop ("for/foreach/while/until/continue"), subroutine, "eval", or
234 "do/require/use"'d file. If more than one value is listed, the list
235 must be placed in parentheses. All listed elements must be legal
236 lvalues. Only alphanumeric identifiers may be lexically
237 scoped--magical built-ins like $/ must currently be "local"ized with
238 "local" instead.
239 Unlike dynamic variables created by the "local" operator, lexical
241 variables declared with "my" are totally hidden from the outside world,
242 including any called subroutines. This is true if it's the same
243 subroutine called from itself or elsewhere--every call gets its own
244 copy.
245 This doesn't mean that a "my" variable declared in a statically
247 enclosing lexical scope would be invisible. Only dynamic scopes are
248 cut off. For example, the "bumpx()" function below has access to the
249 lexical $x variable because both the "my" and the "sub" occurred at the
250 same scope, presumably file scope.
251 my $x = 10;
253 sub bumpx { $x++ }
254 An "eval()", however, can see lexical variables of the scope it is
256 being evaluated in, so long as the names aren't hidden by declarations
257 within the "eval()" itself. See perlref.
258 The parameter list to my() may be assigned to if desired, which allows
260 you to initialize your variables. (If no initializer is given for a
261 particular variable, it is created with the undefined value.) Commonly
262 this is used to name input parameters to a subroutine. Examples:
263 $arg = "fred"; # "global" variable
265 $n = cube_root(27);
266 print "$arg thinks the root is $n\n";
267 fred thinks the root is 3
268 sub cube_root {
270 my $arg = shift; # name doesn't matter
271 $arg **= 1/3;
272 return $arg;
273 }
274 The "my" is simply a modifier on something you might assign to. So
276 when you do assign to variables in its argument list, "my" doesn't
277 change whether those variables are viewed as a scalar or an array. So
278 my ($foo) = <STDIN>; # WRONG?
280 my @FOO = <STDIN>;
281 both supply a list context to the right-hand side, while
283 my $foo = <STDIN>;
285 supplies a scalar context. But the following declares only one
287 variable:
288 my $foo, $bar = 1; # WRONG
290 That has the same effect as
292 my $foo;
294 $bar = 1;
295 The declared variable is not introduced (is not visible) until after
297 the current statement. Thus,
298 my $x = $x;
300 can be used to initialize a new $x with the value of the old $x, and
302 the expression
303 my $x = 123 and $x == 123
305 is false unless the old $x happened to have the value 123.
307 Lexical scopes of control structures are not bounded precisely by the
309 braces that delimit their controlled blocks; control expressions are
310 part of that scope, too. Thus in the loop
311 while (my $line = <>) {
313 $line = lc $line;
314 } continue {
315 print $line;
316 }
317 the scope of $line extends from its declaration throughout the rest of
319 the loop construct (including the "continue" clause), but not beyond
320 it. Similarly, in the conditional
321 if ((my $answer = <STDIN>) =~ /^yes$/i) {
323 user_agrees();
324 } elsif ($answer =~ /^no$/i) {
325 user_disagrees();
326 } else {
327 chomp $answer;
328 die "'$answer' is neither 'yes' nor 'no'";
329 }
330 the scope of $answer extends from its declaration through the rest of
332 that conditional, including any "elsif" and "else" clauses, but not
333 beyond it. See "Simple statements" in perlsyn for information on the
334 scope of variables in statements with modifiers.
335 The "foreach" loop defaults to scoping its index variable dynamically
337 in the manner of "local". However, if the index variable is prefixed
338 with the keyword "my", or if there is already a lexical by that name in
339 scope, then a new lexical is created instead. Thus in the loop
340 for my $i (1, 2, 3) {
342 some_function();
343 }
344 the scope of $i extends to the end of the loop, but not beyond it,
346 rendering the value of $i inaccessible within "some_function()".
347 Some users may wish to encourage the use of lexically scoped variables.
349 As an aid to catching implicit uses to package variables, which are
350 always global, if you say
351 use strict 'vars';
353 then any variable mentioned from there to the end of the enclosing
355 block must either refer to a lexical variable, be predeclared via "our"
356 or "use vars", or else must be fully qualified with the package name.
357 A compilation error results otherwise. An inner block may countermand
358 this with "no strict 'vars'".
359 A "my" has both a compile-time and a run-time effect. At compile time,
361 the compiler takes notice of it. The principal usefulness of this is
362 to quiet "use strict 'vars'", but it is also essential for generation
363 of closures as detailed in perlref. Actual initialization is delayed
364 until run time, though, so it gets executed at the appropriate time,
365 such as each time through a loop, for example.
366 Variables declared with "my" are not part of any package and are
368 therefore never fully qualified with the package name. In particular,
369 you're not allowed to try to make a package variable (or other global)
370 lexical:
371 my $pack::var; # ERROR! Illegal syntax
373 In fact, a dynamic variable (also known as package or global variables)
375 are still accessible using the fully qualified "::" notation even while
376 a lexical of the same name is also visible:
377 package main;
379 local $x = 10;
380 my $x = 20;
381 print "$x and $::x\n";
382 That will print out 20 and 10.
384 You may declare "my" variables at the outermost scope of a file to hide
386 any such identifiers from the world outside that file. This is similar
387 in spirit to C's static variables when they are used at the file level.
388 To do this with a subroutine requires the use of a closure (an
389 anonymous function that accesses enclosing lexicals). If you want to
390 create a private subroutine that cannot be called from outside that
391 block, it can declare a lexical variable containing an anonymous sub
392 reference:
393 my $secret_version = '1.001-beta';
395 my $secret_sub = sub { print $secret_version };
396 &$secret_sub();
397 As long as the reference is never returned by any function within the
399 module, no outside module can see the subroutine, because its name is
400 not in any package's symbol table. Remember that it's not REALLY
401 called $some_pack::secret_version or anything; it's just
402 $secret_version, unqualified and unqualifiable.
403 This does not work with object methods, however; all object methods
405 have to be in the symbol table of some package to be found. See
406 "Function Templates" in perlref for something of a work-around to this.
407 Persistent Private Variables
409 There are two ways to build persistent private variables in Perl 5.10.
410 First, you can simply use the "state" feature. Or, you can use
411 closures, if you want to stay compatible with releases older than 5.10.
412 Persistent variables via state()
414 Beginning with perl 5.9.4, you can declare variables with the "state"
416 keyword in place of "my". For that to work, though, you must have
417 enabled that feature beforehand, either by using the "feature" pragma,
418 or by using "-E" on one-liners. (see feature)
419 For example, the following code maintains a private counter,
421 incremented each time the gimme_another() function is called:
422 use feature 'state';
424 sub gimme_another { state $x; return ++$x }
425 Also, since $x is lexical, it can't be reached or modified by any Perl
427 code outside.
428 When combined with variable declaration, simple scalar assignment to
430 "state" variables (as in "state $x = 42") is executed only the first
431 time. When such statements are evaluated subsequent times, the
432 assignment is ignored. The behavior of this sort of assignment to non-
433 scalar variables is undefined.
434 Persistent variables with closures
436 Just because a lexical variable is lexically (also called statically)
438 scoped to its enclosing block, "eval", or "do" FILE, this doesn't mean
439 that within a function it works like a C static. It normally works
440 more like a C auto, but with implicit garbage collection.
441 Unlike local variables in C or C++, Perl's lexical variables don't
443 necessarily get recycled just because their scope has exited. If
444 something more permanent is still aware of the lexical, it will stick
445 around. So long as something else references a lexical, that lexical
446 won't be freed--which is as it should be. You wouldn't want memory
447 being free until you were done using it, or kept around once you were
448 done. Automatic garbage collection takes care of this for you.
449 This means that you can pass back or save away references to lexical
451 variables, whereas to return a pointer to a C auto is a grave error.
452 It also gives us a way to simulate C's function statics. Here's a
453 mechanism for giving a function private variables with both lexical
454 scoping and a static lifetime. If you do want to create something like
455 C's static variables, just enclose the whole function in an extra
456 block, and put the static variable outside the function but in the
457 block.
458 {
460 my $secret_val = 0;
461 sub gimme_another {
462 return ++$secret_val;
463 }
464 }
465 # $secret_val now becomes unreachable by the outside
466 # world, but retains its value between calls to gimme_another
467 If this function is being sourced in from a separate file via "require"
469 or "use", then this is probably just fine. If it's all in the main
470 program, you'll need to arrange for the "my" to be executed early,
471 either by putting the whole block above your main program, or more
472 likely, placing merely a "BEGIN" code block around it to make sure it
473 gets executed before your program starts to run:
474 BEGIN {
476 my $secret_val = 0;
477 sub gimme_another {
478 return ++$secret_val;
479 }
480 }
481 See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the
483 special triggered code blocks, "BEGIN", "UNITCHECK", "CHECK", "INIT"
484 and "END".
485 If declared at the outermost scope (the file scope), then lexicals work
487 somewhat like C's file statics. They are available to all functions in
488 that same file declared below them, but are inaccessible from outside
489 that file. This strategy is sometimes used in modules to create
490 private variables that the whole module can see.
491 Temporary Values via local()
493 WARNING: In general, you should be using "my" instead of "local",
494 because it's faster and safer. Exceptions to this include the global
495 punctuation variables, global filehandles and formats, and direct
496 manipulation of the Perl symbol table itself. "local" is mostly used
497 when the current value of a variable must be visible to called
498 subroutines.
499 Synopsis:
501 # localization of values
503 local $foo; # make $foo dynamically local
505 local (@wid, %get); # make list of variables local
506 local $foo = "flurp"; # make $foo dynamic, and init it
507 local @oof = @bar; # make @oof dynamic, and init it
508 local $hash{key} = "val"; # sets a local value for this hash entry
510 local ($cond ? $v1 : $v2); # several types of lvalues support
511 # localization
512 # localization of symbols
514 local *FH; # localize $FH, @FH, %FH, &FH ...
516 local *merlyn = *randal; # now $merlyn is really $randal, plus
517 # @merlyn is really @randal, etc
518 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
519 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
520 A "local" modifies its listed variables to be "local" to the enclosing
522 block, "eval", or "do FILE"--and to any subroutine called from within
523 that block. A "local" just gives temporary values to global (meaning
524 package) variables. It does not create a local variable. This is
525 known as dynamic scoping. Lexical scoping is done with "my", which
526 works more like C's auto declarations.
527 Some types of lvalues can be localized as well : hash and array
529 elements and slices, conditionals (provided that their result is always
530 localizable), and symbolic references. As for simple variables, this
531 creates new, dynamically scoped values.
532 If more than one variable or expression is given to "local", they must
534 be placed in parentheses. This operator works by saving the current
535 values of those variables in its argument list on a hidden stack and
536 restoring them upon exiting the block, subroutine, or eval. This means
537 that called subroutines can also reference the local variable, but not
538 the global one. The argument list may be assigned to if desired, which
539 allows you to initialize your local variables. (If no initializer is
540 given for a particular variable, it is created with an undefined
541 value.)
542 Because "local" is a run-time operator, it gets executed each time
544 through a loop. Consequently, it's more efficient to localize your
545 variables outside the loop.
546 Grammatical note on local()
548 A "local" is simply a modifier on an lvalue expression. When you
550 assign to a "local"ized variable, the "local" doesn't change whether
551 its list is viewed as a scalar or an array. So
552 local($foo) = <STDIN>;
554 local @FOO = <STDIN>;
555 both supply a list context to the right-hand side, while
557 local $foo = <STDIN>;
559 supplies a scalar context.
561 Localization of special variables
563 If you localize a special variable, you'll be giving a new value to it,
565 but its magic won't go away. That means that all side-effects related
566 to this magic still work with the localized value.
567 This feature allows code like this to work :
569 # Read the whole contents of FILE in $slurp
571 { local $/ = undef; $slurp = <FILE>; }
572 Note, however, that this restricts localization of some values ; for
574 example, the following statement dies, as of perl 5.9.0, with an error
575 Modification of a read-only value attempted, because the $1 variable is
576 magical and read-only :
577 local $1 = 2;
579 Similarly, but in a way more difficult to spot, the following snippet
581 will die in perl 5.9.0 :
582 sub f { local $_ = "foo"; print }
584 for ($1) {
585 # now $_ is aliased to $1, thus is magic and readonly
586 f();
587 }
588 See next section for an alternative to this situation.
590 WARNING: Localization of tied arrays and hashes does not currently work
592 as described. This will be fixed in a future release of Perl; in the
593 meantime, avoid code that relies on any particular behaviour of
594 localising tied arrays or hashes (localising individual elements is
595 still okay). See "Localising Tied Arrays and Hashes Is Broken" in
596 perl58delta for more details.
597 Localization of globs
599 The construct
601 local *name;
603 creates a whole new symbol table entry for the glob "name" in the
605 current package. That means that all variables in its glob slot
606 ($name, @name, %name, &name, and the "name" filehandle) are dynamically
607 reset.
608 This implies, among other things, that any magic eventually carried by
610 those variables is locally lost. In other words, saying "local */"
611 will not have any effect on the internal value of the input record
612 separator.
613 Notably, if you want to work with a brand new value of the default
615 scalar $_, and avoid the potential problem listed above about $_
616 previously carrying a magic value, you should use "local *_" instead of
617 "local $_". As of perl 5.9.1, you can also use the lexical form of $_
618 (declaring it with "my $_"), which avoids completely this problem.
619 Localization of elements of composite types
621 It's also worth taking a moment to explain what happens when you
623 "local"ize a member of a composite type (i.e. an array or hash
624 element). In this case, the element is "local"ized by name. This means
625 that when the scope of the "local()" ends, the saved value will be
626 restored to the hash element whose key was named in the "local()", or
627 the array element whose index was named in the "local()". If that
628 element was deleted while the "local()" was in effect (e.g. by a
629 "delete()" from a hash or a "shift()" of an array), it will spring back
630 into existence, possibly extending an array and filling in the skipped
631 elements with "undef". For instance, if you say
632 %hash = ( 'This' => 'is', 'a' => 'test' );
634 @ary = ( 0..5 );
635 {
636 local($ary[5]) = 6;
637 local($hash{'a'}) = 'drill';
638 while (my $e = pop(@ary)) {
639 print "$e . . .\n";
640 last unless $e > 3;
641 }
642 if (@ary) {
643 $hash{'only a'} = 'test';
644 delete $hash{'a'};
645 }
646 }
647 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
648 print "The array has ",scalar(@ary)," elements: ",
649 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
650 Perl will print
652 6 . . .
654 4 . . .
655 3 . . .
656 This is a test only a test.
657 The array has 6 elements: 0, 1, 2, undef, undef, 5
658 The behavior of local() on non-existent members of composite types is
660 subject to change in future.
661 Lvalue subroutines
663 WARNING: Lvalue subroutines are still experimental and the
664 implementation may change in future versions of Perl.
665 It is possible to return a modifiable value from a subroutine. To do
667 this, you have to declare the subroutine to return an lvalue.
668 my $val;
670 sub canmod : lvalue {
671 # return $val; this doesn't work, don't say "return"
672 $val;
673 }
674 sub nomod {
675 $val;
676 }
677 canmod() = 5; # assigns to $val
679 nomod() = 5; # ERROR
680 The scalar/list context for the subroutine and for the right-hand side
682 of assignment is determined as if the subroutine call is replaced by a
683 scalar. For example, consider:
684 data(2,3) = get_data(3,4);
686 Both subroutines here are called in a scalar context, while in:
688 (data(2,3)) = get_data(3,4);
690 and in:
692 (data(2),data(3)) = get_data(3,4);
694 all the subroutines are called in a list context.
696 Lvalue subroutines are EXPERIMENTAL
698 They appear to be convenient, but there are several reasons to be
699 circumspect.
700 You can't use the return keyword, you must pass out the value
702 before falling out of subroutine scope. (see comment in example
703 above). This is usually not a problem, but it disallows an
704 explicit return out of a deeply nested loop, which is sometimes a
705 nice way out.
706 They violate encapsulation. A normal mutator can check the
708 supplied argument before setting the attribute it is protecting, an
709 lvalue subroutine never gets that chance. Consider;
710 my $some_array_ref = []; # protected by mutators ??
712 sub set_arr { # normal mutator
714 my $val = shift;
715 die("expected array, you supplied ", ref $val)
716 unless ref $val eq 'ARRAY';
717 $some_array_ref = $val;
718 }
719 sub set_arr_lv : lvalue { # lvalue mutator
720 $some_array_ref;
721 }
722 # set_arr_lv cannot stop this !
724 set_arr_lv() = { a => 1 };
725 Passing Symbol Table Entries (typeglobs)
727 WARNING: The mechanism described in this section was originally the
728 only way to simulate pass-by-reference in older versions of Perl.
729 While it still works fine in modern versions, the new reference
730 mechanism is generally easier to work with. See below.
731 Sometimes you don't want to pass the value of an array to a subroutine
733 but rather the name of it, so that the subroutine can modify the global
734 copy of it rather than working with a local copy. In perl you can
735 refer to all objects of a particular name by prefixing the name with a
736 star: *foo. This is often known as a "typeglob", because the star on
737 the front can be thought of as a wildcard match for all the funny
738 prefix characters on variables and subroutines and such.
739 When evaluated, the typeglob produces a scalar value that represents
741 all the objects of that name, including any filehandle, format, or
742 subroutine. When assigned to, it causes the name mentioned to refer to
743 whatever "*" value was assigned to it. Example:
744 sub doubleary {
746 local(*someary) = @_;
747 foreach $elem (@someary) {
748 $elem *= 2;
749 }
750 }
751 doubleary(*foo);
752 doubleary(*bar);
753 Scalars are already passed by reference, so you can modify scalar
755 arguments without using this mechanism by referring explicitly to $_[0]
756 etc. You can modify all the elements of an array by passing all the
757 elements as scalars, but you have to use the "*" mechanism (or the
758 equivalent reference mechanism) to "push", "pop", or change the size of
759 an array. It will certainly be faster to pass the typeglob (or
760 reference).
761 Even if you don't want to modify an array, this mechanism is useful for
763 passing multiple arrays in a single LIST, because normally the LIST
764 mechanism will merge all the array values so that you can't extract out
765 the individual arrays. For more on typeglobs, see "Typeglobs and
766 Filehandles" in perldata.
767 When to Still Use local()
769 Despite the existence of "my", there are still three places where the
770 "local" operator still shines. In fact, in these three places, you
771 must use "local" instead of "my".
772 1. You need to give a global variable a temporary value, especially
774 $_.
775 The global variables, like @ARGV or the punctuation variables, must
777 be "local"ized with "local()". This block reads in /etc/motd, and
778 splits it up into chunks separated by lines of equal signs, which
779 are placed in @Fields.
780 {
782 local @ARGV = ("/etc/motd");
783 local $/ = undef;
784 local $_ = <>;
785 @Fields = split /^\s*=+\s*$/;
786 }
787 It particular, it's important to "local"ize $_ in any routine that
789 assigns to it. Look out for implicit assignments in "while"
790 conditionals.
791 2. You need to create a local file or directory handle or a local
793 function.
794 A function that needs a filehandle of its own must use "local()" on
796 a complete typeglob. This can be used to create new symbol table
797 entries:
798 sub ioqueue {
800 local (*READER, *WRITER); # not my!
801 pipe (READER, WRITER) or die "pipe: $!";
802 return (*READER, *WRITER);
803 }
804 ($head, $tail) = ioqueue();
805 See the Symbol module for a way to create anonymous symbol table
807 entries.
808 Because assignment of a reference to a typeglob creates an alias,
810 this can be used to create what is effectively a local function, or
811 at least, a local alias.
812 {
814 local *grow = \&shrink; # only until this block exists
815 grow(); # really calls shrink()
816 move(); # if move() grow()s, it shrink()s too
817 }
818 grow(); # get the real grow() again
819 See "Function Templates" in perlref for more about manipulating
821 functions by name in this way.
822 3. You want to temporarily change just one element of an array or
824 hash.
825 You can "local"ize just one element of an aggregate. Usually this
827 is done on dynamics:
828 {
830 local $SIG{INT} = 'IGNORE';
831 funct(); # uninterruptible
832 }
833 # interruptibility automatically restored here
834 But it also works on lexically declared aggregates. Prior to
836 5.005, this operation could on occasion misbehave.
837 Pass by Reference
839 If you want to pass more than one array or hash into a function--or
840 return them from it--and have them maintain their integrity, then
841 you're going to have to use an explicit pass-by-reference. Before you
842 do that, you need to understand references as detailed in perlref.
843 This section may not make much sense to you otherwise.
844 Here are a few simple examples. First, let's pass in several arrays to
846 a function and have it "pop" all of then, returning a new list of all
847 their former last elements:
848 @tailings = popmany ( \@a, \@b, \@c, \@d );
850 sub popmany {
852 my $aref;
853 my @retlist = ();
854 foreach $aref ( @_ ) {
855 push @retlist, pop @$aref;
856 }
857 return @retlist;
858 }
859 Here's how you might write a function that returns a list of keys
861 occurring in all the hashes passed to it:
862 @common = inter( \%foo, \%bar, \%joe );
864 sub inter {
865 my ($k, $href, %seen); # locals
866 foreach $href (@_) {
867 while ( $k = each %$href ) {
868 $seen{$k}++;
869 }
870 }
871 return grep { $seen{$_} == @_ } keys %seen;
872 }
873 So far, we're using just the normal list return mechanism. What
875 happens if you want to pass or return a hash? Well, if you're using
876 only one of them, or you don't mind them concatenating, then the normal
877 calling convention is ok, although a little expensive.
878 Where people get into trouble is here:
880 (@a, @b) = func(@c, @d);
882 or
883 (%a, %b) = func(%c, %d);
884 That syntax simply won't work. It sets just @a or %a and clears the @b
886 or %b. Plus the function didn't get passed into two separate arrays or
887 hashes: it got one long list in @_, as always.
888 If you can arrange for everyone to deal with this through references,
890 it's cleaner code, although not so nice to look at. Here's a function
891 that takes two array references as arguments, returning the two array
892 elements in order of how many elements they have in them:
893 ($aref, $bref) = func(\@c, \@d);
895 print "@$aref has more than @$bref\n";
896 sub func {
897 my ($cref, $dref) = @_;
898 if (@$cref > @$dref) {
899 return ($cref, $dref);
900 } else {
901 return ($dref, $cref);
902 }
903 }
904 It turns out that you can actually do this also:
906 (*a, *b) = func(\@c, \@d);
908 print "@a has more than @b\n";
909 sub func {
910 local (*c, *d) = @_;
911 if (@c > @d) {
912 return (\@c, \@d);
913 } else {
914 return (\@d, \@c);
915 }
916 }
917 Here we're using the typeglobs to do symbol table aliasing. It's a tad
919 subtle, though, and also won't work if you're using "my" variables,
920 because only globals (even in disguise as "local"s) are in the symbol
921 table.
922 If you're passing around filehandles, you could usually just use the
924 bare typeglob, like *STDOUT, but typeglobs references work, too. For
925 example:
926 splutter(\*STDOUT);
928 sub splutter {
929 my $fh = shift;
930 print $fh "her um well a hmmm\n";
931 }
932 $rec = get_rec(\*STDIN);
934 sub get_rec {
935 my $fh = shift;
936 return scalar <$fh>;
937 }
938 If you're planning on generating new filehandles, you could do this.
940 Notice to pass back just the bare *FH, not its reference.
941 sub openit {
943 my $path = shift;
944 local *FH;
945 return open (FH, $path) ? *FH : undef;
946 }
947 Prototypes
949 Perl supports a very limited kind of compile-time argument checking
950 using function prototyping. If you declare
951 sub mypush (\@@)
953 then "mypush()" takes arguments exactly like "push()" does. The
955 function declaration must be visible at compile time. The prototype
956 affects only interpretation of new-style calls to the function, where
957 new-style is defined as not using the "&" character. In other words,
958 if you call it like a built-in function, then it behaves like a built-
959 in function. If you call it like an old-fashioned subroutine, then it
960 behaves like an old-fashioned subroutine. It naturally falls out from
961 this rule that prototypes have no influence on subroutine references
962 like "\&foo" or on indirect subroutine calls like "&{$subref}" or
963 "$subref->()".
964 Method calls are not influenced by prototypes either, because the
966 function to be called is indeterminate at compile time, since the exact
967 code called depends on inheritance.
968 Because the intent of this feature is primarily to let you define
970 subroutines that work like built-in functions, here are prototypes for
971 some other functions that parse almost exactly like the corresponding
972 built-in.
973 Declared as Called as
975 sub mylink ($$) mylink $old, $new
977 sub myvec ($$$) myvec $var, $offset, 1
978 sub myindex ($$;$) myindex &getstring, "substr"
979 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
980 sub myreverse (@) myreverse $a, $b, $c
981 sub myjoin ($@) myjoin ":", $a, $b, $c
982 sub mypop (\@) mypop @array
983 sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme
984 sub mykeys (\%) mykeys %{$hashref}
985 sub myopen (*;$) myopen HANDLE, $name
986 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
987 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
988 sub myrand (;$) myrand 42
989 sub mytime () mytime
990 Any backslashed prototype character represents an actual argument that
992 absolutely must start with that character. The value passed as part of
993 @_ will be a reference to the actual argument given in the subroutine
994 call, obtained by applying "\" to that argument.
995 You can also backslash several argument types simultaneously by using
997 the "\[]" notation:
998 sub myref (\[$@%&*])
1000 will allow calling myref() as
1002 myref $var
1004 myref @array
1005 myref %hash
1006 myref &sub
1007 myref *glob
1008 and the first argument of myref() will be a reference to a scalar, an
1010 array, a hash, a code, or a glob.
1011 Unbackslashed prototype characters have special meanings. Any
1013 unbackslashed "@" or "%" eats all remaining arguments, and forces list
1014 context. An argument represented by "$" forces scalar context. An "&"
1015 requires an anonymous subroutine, which, if passed as the first
1016 argument, does not require the "sub" keyword or a subsequent comma.
1017 A "*" allows the subroutine to accept a bareword, constant, scalar
1019 expression, typeglob, or a reference to a typeglob in that slot. The
1020 value will be available to the subroutine either as a simple scalar, or
1021 (in the latter two cases) as a reference to the typeglob. If you wish
1022 to always convert such arguments to a typeglob reference, use
1023 Symbol::qualify_to_ref() as follows:
1024 use Symbol 'qualify_to_ref';
1026 sub foo (*) {
1028 my $fh = qualify_to_ref(shift, caller);
1029 ...
1030 }
1031 A semicolon (";") separates mandatory arguments from optional
1033 arguments. It is redundant before "@" or "%", which gobble up
1034 everything else.
1035 As the last character of a prototype, or just before a semicolon, you
1037 can use "_" in place of "$": if this argument is not provided, $_ will
1038 be used instead.
1039 Note how the last three examples in the table above are treated
1041 specially by the parser. "mygrep()" is parsed as a true list operator,
1042 "myrand()" is parsed as a true unary operator with unary precedence the
1043 same as "rand()", and "mytime()" is truly without arguments, just like
1044 "time()". That is, if you say
1045 mytime +2;
1047 you'll get "mytime() + 2", not mytime(2), which is how it would be
1049 parsed without a prototype.
1050 The interesting thing about "&" is that you can generate new syntax
1052 with it, provided it's in the initial position:
1053 sub try (&@) {
1055 my($try,$catch) = @_;
1056 eval { &$try };
1057 if ($@) {
1058 local $_ = $@;
1059 &$catch;
1060 }
1061 }
1062 sub catch (&) { $_[0] }
1063 try {
1065 die "phooey";
1066 } catch {
1067 /phooey/ and print "unphooey\n";
1068 };
1069 That prints "unphooey". (Yes, there are still unresolved issues having
1071 to do with visibility of @_. I'm ignoring that question for the
1072 moment. (But note that if we make @_ lexically scoped, those anonymous
1073 subroutines can act like closures... (Gee, is this sounding a little
1074 Lispish? (Never mind.))))
1075 And here's a reimplementation of the Perl "grep" operator:
1077 sub mygrep (&@) {
1079 my $code = shift;
1080 my @result;
1081 foreach $_ (@_) {
1082 push(@result, $_) if &$code;
1083 }
1084 @result;
1085 }
1086 Some folks would prefer full alphanumeric prototypes. Alphanumerics
1088 have been intentionally left out of prototypes for the express purpose
1089 of someday in the future adding named, formal parameters. The current
1090 mechanism's main goal is to let module writers provide better
1091 diagnostics for module users. Larry feels the notation quite
1092 understandable to Perl programmers, and that it will not intrude
1093 greatly upon the meat of the module, nor make it harder to read. The
1094 line noise is visually encapsulated into a small pill that's easy to
1095 swallow.
1096 If you try to use an alphanumeric sequence in a prototype you will
1098 generate an optional warning - "Illegal character in prototype...".
1099 Unfortunately earlier versions of Perl allowed the prototype to be used
1100 as long as its prefix was a valid prototype. The warning may be
1101 upgraded to a fatal error in a future version of Perl once the majority
1102 of offending code is fixed.
1103 It's probably best to prototype new functions, not retrofit prototyping
1105 into older ones. That's because you must be especially careful about
1106 silent impositions of differing list versus scalar contexts. For
1107 example, if you decide that a function should take just one parameter,
1108 like this:
1109 sub func ($) {
1111 my $n = shift;
1112 print "you gave me $n\n";
1113 }
1114 and someone has been calling it with an array or expression returning a
1116 list:
1117 func(@foo);
1119 func( split /:/ );
1120 Then you've just supplied an automatic "scalar" in front of their
1122 argument, which can be more than a bit surprising. The old @foo which
1123 used to hold one thing doesn't get passed in. Instead, "func()" now
1124 gets passed in a 1; that is, the number of elements in @foo. And the
1125 "split" gets called in scalar context so it starts scribbling on your
1126 @_ parameter list. Ouch!
1127 This is all very powerful, of course, and should be used only in
1129 moderation to make the world a better place.
1130 Constant Functions
1132 Functions with a prototype of "()" are potential candidates for
1133 inlining. If the result after optimization and constant folding is
1134 either a constant or a lexically-scoped scalar which has no other
1135 references, then it will be used in place of function calls made
1136 without "&". Calls made using "&" are never inlined. (See constant.pm
1137 for an easy way to declare most constants.)
1138 The following functions would all be inlined:
1140 sub pi () { 3.14159 } # Not exact, but close.
1142 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1143 # and it's inlined, too!
1144 sub ST_DEV () { 0 }
1145 sub ST_INO () { 1 }
1146 sub FLAG_FOO () { 1 << 8 }
1148 sub FLAG_BAR () { 1 << 9 }
1149 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1150 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1152 sub N () { int(OPT_BAZ) / 3 }
1154 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1156 Be aware that these will not be inlined; as they contain inner scopes,
1158 the constant folding doesn't reduce them to a single constant:
1159 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1161 sub baz_val () {
1163 if (OPT_BAZ) {
1164 return 23;
1165 }
1166 else {
1167 return 42;
1168 }
1169 }
1170 If you redefine a subroutine that was eligible for inlining, you'll get
1172 a mandatory warning. (You can use this warning to tell whether or not
1173 a particular subroutine is considered constant.) The warning is
1174 considered severe enough not to be optional because previously compiled
1175 invocations of the function will still be using the old value of the
1176 function. If you need to be able to redefine the subroutine, you need
1177 to ensure that it isn't inlined, either by dropping the "()" prototype
1178 (which changes calling semantics, so beware) or by thwarting the
1179 inlining mechanism in some other way, such as
1180 sub not_inlined () {
1182 23 if $];
1183 }
1184 Overriding Built-in Functions
1186 Many built-in functions may be overridden, though this should be tried
1187 only occasionally and for good reason. Typically this might be done by
1188 a package attempting to emulate missing built-in functionality on a
1189 non-Unix system.
1190 Overriding may be done only by importing the name from a module at
1192 compile time--ordinary predeclaration isn't good enough. However, the
1193 "use subs" pragma lets you, in effect, predeclare subs via the import
1194 syntax, and these names may then override built-in ones:
1195 use subs 'chdir', 'chroot', 'chmod', 'chown';
1197 chdir $somewhere;
1198 sub chdir { ... }
1199 To unambiguously refer to the built-in form, precede the built-in name
1201 with the special package qualifier "CORE::". For example, saying
1202 "CORE::open()" always refers to the built-in "open()", even if the
1203 current package has imported some other subroutine called "&open()"
1204 from elsewhere. Even though it looks like a regular function call, it
1205 isn't: you can't take a reference to it, such as the incorrect
1206 "\&CORE::open" might appear to produce.
1207 Library modules should not in general export built-in names like "open"
1209 or "chdir" as part of their default @EXPORT list, because these may
1210 sneak into someone else's namespace and change the semantics
1211 unexpectedly. Instead, if the module adds that name to @EXPORT_OK,
1212 then it's possible for a user to import the name explicitly, but not
1213 implicitly. That is, they could say
1214 use Module 'open';
1216 and it would import the "open" override. But if they said
1218 use Module;
1220 they would get the default imports without overrides.
1222 The foregoing mechanism for overriding built-in is restricted, quite
1224 deliberately, to the package that requests the import. There is a
1225 second method that is sometimes applicable when you wish to override a
1226 built-in everywhere, without regard to namespace boundaries. This is
1227 achieved by importing a sub into the special namespace
1228 "CORE::GLOBAL::". Here is an example that quite brazenly replaces the
1229 "glob" operator with something that understands regular expressions.
1230 package REGlob;
1232 require Exporter;
1233 @ISA = 'Exporter';
1234 @EXPORT_OK = 'glob';
1235 sub import {
1237 my $pkg = shift;
1238 return unless @_;
1239 my $sym = shift;
1240 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1241 $pkg->export($where, $sym, @_);
1242 }
1243 sub glob {
1245 my $pat = shift;
1246 my @got;
1247 if (opendir my $d, '.') {
1248 @got = grep /$pat/, readdir $d;
1249 closedir $d;
1250 }
1251 return @got;
1252 }
1253 1;
1254 And here's how it could be (ab)used:
1256 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1258 package Foo;
1259 use REGlob 'glob'; # override glob() in Foo:: only
1260 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1261 The initial comment shows a contrived, even dangerous example. By
1263 overriding "glob" globally, you would be forcing the new (and
1264 subversive) behavior for the "glob" operator for every namespace,
1265 without the complete cognizance or cooperation of the modules that own
1266 those namespaces. Naturally, this should be done with extreme
1267 caution--if it must be done at all.
1268 The "REGlob" example above does not implement all the support needed to
1270 cleanly override perl's "glob" operator. The built-in "glob" has
1271 different behaviors depending on whether it appears in a scalar or list
1272 context, but our "REGlob" doesn't. Indeed, many perl built-in have
1273 such context sensitive behaviors, and these must be adequately
1274 supported by a properly written override. For a fully functional
1275 example of overriding "glob", study the implementation of
1276 "File::DosGlob" in the standard library.
1277 When you override a built-in, your replacement should be consistent (if
1279 possible) with the built-in native syntax. You can achieve this by
1280 using a suitable prototype. To get the prototype of an overridable
1281 built-in, use the "prototype" function with an argument of
1282 "CORE::builtin_name" (see "prototype" in perlfunc).
1283 Note however that some built-ins can't have their syntax expressed by a
1285 prototype (such as "system" or "chomp"). If you override them you
1286 won't be able to fully mimic their original syntax.
1287 The built-ins "do", "require" and "glob" can also be overridden, but
1289 due to special magic, their original syntax is preserved, and you don't
1290 have to define a prototype for their replacements. (You can't override
1291 the "do BLOCK" syntax, though).
1292 "require" has special additional dark magic: if you invoke your
1294 "require" replacement as "require Foo::Bar", it will actually receive
1295 the argument "Foo/Bar.pm" in @_. See "require" in perlfunc.
1296 And, as you'll have noticed from the previous example, if you override
1298 "glob", the "<*>" glob operator is overridden as well.
1299 In a similar fashion, overriding the "readline" function also overrides
1301 the equivalent I/O operator "<FILEHANDLE>". Also, overriding "readpipe"
1302 also overrides the operators "``" and "qx//".
1303 Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.
1305 Autoloading
1307 If you call a subroutine that is undefined, you would ordinarily get an
1308 immediate, fatal error complaining that the subroutine doesn't exist.
1309 (Likewise for subroutines being used as methods, when the method
1310 doesn't exist in any base class of the class's package.) However, if
1311 an "AUTOLOAD" subroutine is defined in the package or packages used to
1312 locate the original subroutine, then that "AUTOLOAD" subroutine is
1313 called with the arguments that would have been passed to the original
1314 subroutine. The fully qualified name of the original subroutine
1315 magically appears in the global $AUTOLOAD variable of the same package
1316 as the "AUTOLOAD" routine. The name is not passed as an ordinary
1317 argument because, er, well, just because, that's why. (As an
1318 exception, a method call to a nonexistent "import" or "unimport" method
1319 is just skipped instead.)
1320 Many "AUTOLOAD" routines load in a definition for the requested
1322 subroutine using eval(), then execute that subroutine using a special
1323 form of goto() that erases the stack frame of the "AUTOLOAD" routine
1324 without a trace. (See the source to the standard module documented in
1325 AutoLoader, for example.) But an "AUTOLOAD" routine can also just
1326 emulate the routine and never define it. For example, let's pretend
1327 that a function that wasn't defined should just invoke "system" with
1328 those arguments. All you'd do is:
1329 sub AUTOLOAD {
1331 my $program = $AUTOLOAD;
1332 $program =~ s/.*:://;
1333 system($program, @_);
1334 }
1335 date();
1336 who('am', 'i');
1337 ls('-l');
1338 In fact, if you predeclare functions you want to call that way, you
1340 don't even need parentheses:
1341 use subs qw(date who ls);
1343 date;
1344 who "am", "i";
1345 ls '-l';
1346 A more complete example of this is the standard Shell module, which can
1348 treat undefined subroutine calls as calls to external programs.
1349 Mechanisms are available to help modules writers split their modules
1351 into autoloadable files. See the standard AutoLoader module described
1352 in AutoLoader and in AutoSplit, the standard SelfLoader modules in
1353 SelfLoader, and the document on adding C functions to Perl code in
1354 perlxs.
1355 Subroutine Attributes
1357 A subroutine declaration or definition may have a list of attributes
1358 associated with it. If such an attribute list is present, it is broken
1359 up at space or colon boundaries and treated as though a "use
1360 attributes" had been seen. See attributes for details about what
1361 attributes are currently supported. Unlike the limitation with the
1362 obsolescent "use attrs", the "sub : ATTRLIST" syntax works to associate
1363 the attributes with a pre-declaration, and not just with a subroutine
1364 definition.
1365 The attributes must be valid as simple identifier names (without any
1367 punctuation other than the '_' character). They may have a parameter
1368 list appended, which is only checked for whether its parentheses
1369 ('(',')') nest properly.
1370 Examples of valid syntax (even though the attributes are unknown):
1372 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1374 sub plugh () : Ugly('\(") :Bad;
1375 sub xyzzy : _5x5 { ... }
1376 Examples of invalid syntax:
1378 sub fnord : switch(10,foo(); # ()-string not balanced
1380 sub snoid : Ugly('('); # ()-string not balanced
1381 sub xyzzy : 5x5; # "5x5" not a valid identifier
1382 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1383 sub snurt : foo + bar; # "+" not a colon or space
1384 The attribute list is passed as a list of constant strings to the code
1386 which associates them with the subroutine. In particular, the second
1387 example of valid syntax above currently looks like this in terms of how
1388 it's parsed and invoked:
1389 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1391 For further details on attribute lists and their manipulation, see
1393 attributes and Attribute::Handlers.
1394
1396 See "Function Templates" in perlref for more about references and
1397 closures. See perlxs if you'd like to learn about calling C
1398 subroutines from Perl. See perlembed if you'd like to learn about
1399 calling Perl subroutines from C. See perlmod to learn about bundling
1400 up your functions in separate files. See perlmodlib to learn what
1401 library modules come standard on your system. See perltoot to learn
1402 how to make object method calls.
1403perl v5.10.1 2009-02-12 PERLSUB(1)