1feature(3pm) Perl Programmers Reference Guide feature(3pm)
2
6 feature - Perl pragma to enable new features
7
9 use feature qw(fc say);
10 # Without the "use feature" above, this code would not be able to find
12 # the built-ins "say" or "fc":
13 say "The case-folded version of $x is: " . fc $x;
14 # set features to match the :5.10 bundle, which may turn off or on
17 # multiple features (see below)
18 use feature ':5.10';
19 # implicitly loads :5.10 feature bundle
22 use v5.10;
23
25 It is usually impossible to add new syntax to Perl without breaking
26 some existing programs. This pragma provides a way to minimize that
27 risk. New syntactic constructs, or new semantic meanings to older
28 constructs, can be enabled by "use feature 'foo'", and will be parsed
29 only when the appropriate feature pragma is in scope. (Nevertheless,
30 the "CORE::" prefix provides access to all Perl keywords, regardless of
31 this pragma.)
32 Lexical effect
34 Like other pragmas ("use strict", for example), features have a lexical
35 effect. "use feature qw(foo)" will only make the feature "foo"
36 available from that point to the end of the enclosing block.
37 {
39 use feature 'say';
40 say "say is available here";
41 }
42 print "But not here.\n";
43 "no feature"
45 Features can also be turned off by using "no feature "foo"". This too
46 has lexical effect.
47 use feature 'say';
49 say "say is available here";
50 {
51 no feature 'say';
52 print "But not here.\n";
53 }
54 say "Yet it is here.";
55 "no feature" with no features specified will reset to the default
57 group. To disable all features (an unusual request!) use "no feature
58 ':all'".
59
61 The 'say' feature
62 "use feature 'say'" tells the compiler to enable the Raku-inspired
63 "say" function.
64 See "say" in perlfunc for details.
66 This feature is available starting with Perl 5.10.
68 The 'state' feature
70 "use feature 'state'" tells the compiler to enable "state" variables.
71 See "Persistent Private Variables" in perlsub for details.
73 This feature is available starting with Perl 5.10.
75 The 'switch' feature
77 WARNING: This feature is still experimental and the implementation may
78 change or be removed in future versions of Perl. For this reason, Perl
79 will warn when you use the feature, unless you have explicitly disabled
80 the warning:
81 no warnings "experimental::smartmatch";
83 "use feature 'switch'" tells the compiler to enable the Raku given/when
85 construct.
86 See "Switch Statements" in perlsyn for details.
88 This feature is available starting with Perl 5.10.
90 The 'unicode_strings' feature
92 "use feature 'unicode_strings'" tells the compiler to use Unicode rules
93 in all string operations executed within its scope (unless they are
94 also within the scope of either "use locale" or "use bytes"). The same
95 applies to all regular expressions compiled within the scope, even if
96 executed outside it. It does not change the internal representation of
97 strings, but only how they are interpreted.
98 "no feature 'unicode_strings'" tells the compiler to use the
100 traditional Perl rules wherein the native character set rules is used
101 unless it is clear to Perl that Unicode is desired. This can lead to
102 some surprises when the behavior suddenly changes. (See "The "Unicode
103 Bug"" in perlunicode for details.) For this reason, if you are
104 potentially using Unicode in your program, the "use feature
105 'unicode_strings'" subpragma is strongly recommended.
106 This feature is available starting with Perl 5.12; was almost fully
108 implemented in Perl 5.14; and extended in Perl 5.16 to cover
109 "quotemeta"; was extended further in Perl 5.26 to cover the range
110 operator; and was extended again in Perl 5.28 to cover special-cased
111 whitespace splitting.
112 The 'unicode_eval' and 'evalbytes' features
114 Together, these two features are intended to replace the legacy string
115 "eval" function, which behaves problematically in some instances. They
116 are available starting with Perl 5.16, and are enabled by default by a
117 "use 5.16" or higher declaration.
118 "unicode_eval" changes the behavior of plain string "eval" to work more
120 consistently, especially in the Unicode world. Certain (mis)behaviors
121 couldn't be changed without breaking some things that had come to rely
122 on them, so the feature can be enabled and disabled. Details are at
123 "Under the "unicode_eval" feature" in perlfunc.
124 "evalbytes" is like string "eval", but operating on a byte stream that
126 is not UTF-8 encoded. Details are at "evalbytes EXPR" in perlfunc.
127 Without a "use feature 'evalbytes'" nor a "use v5.16" (or higher)
128 declaration in the current scope, you can still access it by instead
129 writing "CORE::evalbytes".
130 The 'current_sub' feature
132 This provides the "__SUB__" token that returns a reference to the
133 current subroutine or "undef" outside of a subroutine.
134 This feature is available starting with Perl 5.16.
136 The 'array_base' feature
138 This feature supported the legacy $[ variable. See "$[" in perlvar.
139 It was on by default but disabled under "use v5.16" (see "IMPLICIT
140 LOADING", below) and unavailable since perl 5.30.
141 This feature is available under this name starting with Perl 5.16. In
143 previous versions, it was simply on all the time, and this pragma knew
144 nothing about it.
145 The 'fc' feature
147 "use feature 'fc'" tells the compiler to enable the "fc" function,
148 which implements Unicode casefolding.
149 See "fc" in perlfunc for details.
151 This feature is available from Perl 5.16 onwards.
153 The 'lexical_subs' feature
155 In Perl versions prior to 5.26, this feature enabled declaration of
156 subroutines via "my sub foo", "state sub foo" and "our sub foo" syntax.
157 See "Lexical Subroutines" in perlsub for details.
158 This feature is available from Perl 5.18 onwards. From Perl 5.18 to
160 5.24, it was classed as experimental, and Perl emitted a warning for
161 its usage, except when explicitly disabled:
162 no warnings "experimental::lexical_subs";
164 As of Perl 5.26, use of this feature no longer triggers a warning,
166 though the "experimental::lexical_subs" warning category still exists
167 (for compatibility with code that disables it). In addition, this
168 syntax is not only no longer experimental, but it is enabled for all
169 Perl code, regardless of what feature declarations are in scope.
170 The 'postderef' and 'postderef_qq' features
172 The 'postderef_qq' feature extends the applicability of postfix
173 dereference syntax so that postfix array and scalar dereference are
174 available in double-quotish interpolations. For example, it makes the
175 following two statements equivalent:
176 my $s = "[@{ $h->{a} }]";
178 my $s = "[$h->{a}->@*]";
179 This feature is available from Perl 5.20 onwards. In Perl 5.20 and
181 5.22, it was classed as experimental, and Perl emitted a warning for
182 its usage, except when explicitly disabled:
183 no warnings "experimental::postderef";
185 As of Perl 5.24, use of this feature no longer triggers a warning,
187 though the "experimental::postderef" warning category still exists (for
188 compatibility with code that disables it).
189 The 'postderef' feature was used in Perl 5.20 and Perl 5.22 to enable
191 postfix dereference syntax outside double-quotish interpolations. In
192 those versions, using it triggered the "experimental::postderef"
193 warning in the same way as the 'postderef_qq' feature did. As of Perl
194 5.24, this syntax is not only no longer experimental, but it is enabled
195 for all Perl code, regardless of what feature declarations are in
196 scope.
197 The 'signatures' feature
199 WARNING: This feature is still experimental and the implementation may
200 change or be removed in future versions of Perl. For this reason, Perl
201 will warn when you use the feature, unless you have explicitly disabled
202 the warning:
203 no warnings "experimental::signatures";
205 This enables unpacking of subroutine arguments into lexical variables
207 by syntax such as
208 sub foo ($left, $right) {
210 return $left + $right;
211 }
212 See "Signatures" in perlsub for details.
214 This feature is available from Perl 5.20 onwards.
216 The 'refaliasing' feature
218 WARNING: This feature is still experimental and the implementation may
219 change or be removed in future versions of Perl. For this reason, Perl
220 will warn when you use the feature, unless you have explicitly disabled
221 the warning:
222 no warnings "experimental::refaliasing";
224 This enables aliasing via assignment to references:
226 \$a = \$b; # $a and $b now point to the same scalar
228 \@a = \@b; # to the same array
229 \%a = \%b;
230 \&a = \&b;
231 foreach \%hash (@array_of_hash_refs) {
232 ...
233 }
234 See "Assigning to References" in perlref for details.
236 This feature is available from Perl 5.22 onwards.
238 The 'bitwise' feature
240 This makes the four standard bitwise operators ("& | ^ ~") treat their
241 operands consistently as numbers, and introduces four new dotted
242 operators ("&. |. ^. ~.") that treat their operands consistently as
243 strings. The same applies to the assignment variants ("&= |= ^= &.=
244 |.= ^.=").
245 See "Bitwise String Operators" in perlop for details.
247 This feature is available from Perl 5.22 onwards. Starting in Perl
249 5.28, "use v5.28" will enable the feature. Before 5.28, it was still
250 experimental and would emit a warning in the "experimental::bitwise"
251 category.
252 The 'declared_refs' feature
254 WARNING: This feature is still experimental and the implementation may
255 change or be removed in future versions of Perl. For this reason, Perl
256 will warn when you use the feature, unless you have explicitly disabled
257 the warning:
258 no warnings "experimental::declared_refs";
260 This allows a reference to a variable to be declared with "my",
262 "state", our "our", or localized with "local". It is intended mainly
263 for use in conjunction with the "refaliasing" feature. See "Declaring
264 a Reference to a Variable" in perlref for examples.
265 This feature is available from Perl 5.26 onwards.
267 The 'isa' feature
269 WARNING: This feature is still experimental and the implementation may
270 change or be removed in future versions of Perl. For this reason, Perl
271 will warn when you use the feature, unless you have explicitly disabled
272 the warning:
273 no warnings "experimental::isa";
275 This allows the use of the "isa" infix operator, which tests whether
277 the scalar given by the left operand is an object of the class given by
278 the right operand. See "Class Instance Operator" in perlop for more
279 details.
280 This feature is available from Perl 5.32 onwards.
282 The 'indirect' feature
284 This feature allows the use of indirect object syntax for method calls,
285 e.g. "new Foo 1, 2;". It is enabled by default, but can be turned off
286 to disallow indirect object syntax.
287 This feature is available under this name from Perl 5.32 onwards. In
289 previous versions, it was simply on all the time. To disallow (or warn
290 on) indirect object syntax on older Perls, see the indirect CPAN
291 module.
292 The 'multidimensional' feature
294 This feature enables multidimensional array emulation, a perl 4 (or
295 earlier) feature that was used to emulate multidimensional arrays with
296 hashes. This works by converting code like $foo{$x, $y} into
297 $foo{join($;, $x, $y)}. It is enabled by default, but can be turned
298 off to disable multidimensional array emulation.
299 When this feature is disabled the syntax that is normally replaced will
301 report a compilation error.
302 This feature is available under this name from Perl 5.34 onwards. In
304 previous versions, it was simply on all the time.
305 You can use the multidimensional module on CPAN to disable
307 multidimensional array emulation for older versions of Perl.
308 The 'bareword_filehandles' feature.
310 This feature enables bareword filehandles for builtin functions
311 operations, a generally discouraged practice. It is enabled by
312 default, but can be turned off to disable bareword filehandles, except
313 for the exceptions listed below.
314 The perl built-in filehandles "STDIN", "STDOUT", "STDERR", "DATA",
316 "ARGV", "ARGVOUT" and the special "_" are always enabled.
317 This feature is enabled under this name from Perl 5.34 onwards. In
319 previous versions it was simply on all the time.
320 You can use the bareword::filehandles module on CPAN to disable
322 bareword filehandles for older versions of perl.
323 The 'try' feature.
325 WARNING: This feature is still experimental and the implementation may
326 change or be removed in future versions of Perl. For this reason, Perl
327 will warn when you use the feature, unless you have explicitly disabled
328 the warning:
329 no warnings "experimental::try";
331 This feature enables the "try" and "catch" syntax, which allows
333 exception handling, where exceptions thrown from the body of the block
334 introduced with "try" are caught by executing the body of the "catch"
335 block.
336 For more information, see "Try Catch Exception Handling" in perlsyn.
338
340 It's possible to load multiple features together, using a feature
341 bundle. The name of a feature bundle is prefixed with a colon, to
342 distinguish it from an actual feature.
343 use feature ":5.10";
345 The following feature bundles are available:
347 bundle features included
349 --------- -----------------
350 :default indirect multidimensional
351 bareword_filehandles
352 :5.10 bareword_filehandles indirect
354 multidimensional say state switch
355 :5.12 bareword_filehandles indirect
357 multidimensional say state switch
358 unicode_strings
359 :5.14 bareword_filehandles indirect
361 multidimensional say state switch
362 unicode_strings
363 :5.16 bareword_filehandles current_sub evalbytes
365 fc indirect multidimensional say state
366 switch unicode_eval unicode_strings
367 :5.18 bareword_filehandles current_sub evalbytes
369 fc indirect multidimensional say state
370 switch unicode_eval unicode_strings
371 :5.20 bareword_filehandles current_sub evalbytes
373 fc indirect multidimensional say state
374 switch unicode_eval unicode_strings
375 :5.22 bareword_filehandles current_sub evalbytes
377 fc indirect multidimensional say state
378 switch unicode_eval unicode_strings
379 :5.24 bareword_filehandles current_sub evalbytes
381 fc indirect multidimensional postderef_qq
382 say state switch unicode_eval
383 unicode_strings
384 :5.26 bareword_filehandles current_sub evalbytes
386 fc indirect multidimensional postderef_qq
387 say state switch unicode_eval
388 unicode_strings
389 :5.28 bareword_filehandles bitwise current_sub
391 evalbytes fc indirect multidimensional
392 postderef_qq say state switch unicode_eval
393 unicode_strings
394 :5.30 bareword_filehandles bitwise current_sub
396 evalbytes fc indirect multidimensional
397 postderef_qq say state switch unicode_eval
398 unicode_strings
399 :5.32 bareword_filehandles bitwise current_sub
401 evalbytes fc indirect multidimensional
402 postderef_qq say state switch unicode_eval
403 unicode_strings
404 :5.34 bareword_filehandles bitwise current_sub
406 evalbytes fc indirect multidimensional
407 postderef_qq say state switch unicode_eval
408 unicode_strings
409 The ":default" bundle represents the feature set that is enabled before
411 any "use feature" or "no feature" declaration.
412 Specifying sub-versions such as the 0 in 5.14.0 in feature bundles has
414 no effect. Feature bundles are guaranteed to be the same for all sub-
415 versions.
416 use feature ":5.14.0"; # same as ":5.14"
418 use feature ":5.14.1"; # same as ":5.14"
419
421 Instead of loading feature bundles by name, it is easier to let Perl do
422 implicit loading of a feature bundle for you.
423 There are two ways to load the "feature" pragma implicitly:
425 • By using the "-E" switch on the Perl command-line instead of "-e".
427 That will enable the feature bundle for that version of Perl in the
428 main compilation unit (that is, the one-liner that follows "-E").
429 • By explicitly requiring a minimum Perl version number for your
431 program, with the "use VERSION" construct. That is,
432 use v5.10.0;
434 will do an implicit
436 no feature ':all';
438 use feature ':5.10';
439 and so on. Note how the trailing sub-version is automatically
441 stripped from the version.
442 But to avoid portability warnings (see "use" in perlfunc), you may
444 prefer:
445 use 5.010;
447 with the same effect.
449 If the required version is older than Perl 5.10, the ":default"
451 feature bundle is automatically loaded instead.
452 Unlike "use feature ":5.12"", saying "use v5.12" (or any higher
454 version) also does the equivalent of "use strict"; see "use" in
455 perlfunc for details.
456perl v5.34.0 2021-10-18 feature(3pm)