1feature(3pm) Perl Programmers Reference Guide feature(3pm)
2
6 feature - Perl pragma to enable new features
7
9 use feature qw(say switch);
10 given ($foo) {
11 when (1) { say "\$foo == 1" }
12 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
13 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
14 when ($_ > 100) { say "\$foo > 100" }
15 default { say "None of the above" }
16 }
17 use feature ':5.10'; # loads all features available in perl 5.10
19 use v5.10; # implicitly loads :5.10 feature bundle
21
23 It is usually impossible to add new syntax to Perl without breaking
24 some existing programs. This pragma provides a way to minimize that
25 risk. New syntactic constructs, or new semantic meanings to older
26 constructs, can be enabled by "use feature 'foo'", and will be parsed
27 only when the appropriate feature pragma is in scope. (Nevertheless,
28 the "CORE::" prefix provides access to all Perl keywords, regardless of
29 this pragma.)
30 Lexical effect
32 Like other pragmas ("use strict", for example), features have a lexical
33 effect. "use feature qw(foo)" will only make the feature "foo"
34 available from that point to the end of the enclosing block.
35 {
37 use feature 'say';
38 say "say is available here";
39 }
40 print "But not here.\n";
41 "no feature"
43 Features can also be turned off by using "no feature "foo"". This too
44 has lexical effect.
45 use feature 'say';
47 say "say is available here";
48 {
49 no feature 'say';
50 print "But not here.\n";
51 }
52 say "Yet it is here.";
53 "no feature" with no features specified will reset to the default
55 group. To disable all features (an unusual request!) use "no feature
56 ':all'".
57
59 The 'say' feature
60 "use feature 'say'" tells the compiler to enable the Perl 6 style "say"
61 function.
62 See "say" in perlfunc for details.
64 This feature is available starting with Perl 5.10.
66 The 'state' feature
68 "use feature 'state'" tells the compiler to enable "state" variables.
69 See "Persistent Private Variables" in perlsub for details.
71 This feature is available starting with Perl 5.10.
73 The 'switch' feature
75 WARNING: Because the smartmatch operator is experimental, Perl will
76 warn when you use this feature, unless you have explicitly disabled the
77 warning:
78 no warnings "experimental::smartmatch";
80 "use feature 'switch'" tells the compiler to enable the Perl 6
82 given/when construct.
83 See "Switch Statements" in perlsyn for details.
85 This feature is available starting with Perl 5.10.
87 The 'unicode_strings' feature
89 "use feature 'unicode_strings'" tells the compiler to use Unicode rules
90 in all string operations executed within its scope (unless they are
91 also within the scope of either "use locale" or "use bytes"). The same
92 applies to all regular expressions compiled within the scope, even if
93 executed outside it. It does not change the internal representation of
94 strings, but only how they are interpreted.
95 "no feature 'unicode_strings'" tells the compiler to use the
97 traditional Perl rules wherein the native character set rules is used
98 unless it is clear to Perl that Unicode is desired. This can lead to
99 some surprises when the behavior suddenly changes. (See "The "Unicode
100 Bug"" in perlunicode for details.) For this reason, if you are
101 potentially using Unicode in your program, the "use feature
102 'unicode_strings'" subpragma is strongly recommended.
103 This feature is available starting with Perl 5.12; was almost fully
105 implemented in Perl 5.14; and extended in Perl 5.16 to cover
106 "quotemeta"; was extended further in Perl 5.26 to cover the range
107 operator; and was extended again in Perl 5.28 to cover special-cased
108 whitespace splitting.
109 The 'unicode_eval' and 'evalbytes' features
111 Together, these two features are intended to replace the legacy string
112 "eval" function, which behaves problematically in some instances. They
113 are available starting with Perl 5.16, and are enabled by default by a
114 "use 5.16" or higher declaration.
115 "unicode_eval" changes the behavior of plain string "eval" to work more
117 consistently, especially in the Unicode world. Certain (mis)behaviors
118 couldn't be changed without breaking some things that had come to rely
119 on them, so the feature can be enabled and disabled. Details are at
120 "Under the "unicode_eval" feature" in perlfunc.
121 "evalbytes" is like string "eval", but operating on a byte stream that
123 is not UTF-8 encoded. Details are at "evalbytes EXPR" in perlfunc.
124 Without a "use feature 'evalbytes'" nor a "use v5.16" (or higher)
125 declaration in the current scope, you can still access it by instead
126 writing "CORE::evalbytes".
127 The 'current_sub' feature
129 This provides the "__SUB__" token that returns a reference to the
130 current subroutine or "undef" outside of a subroutine.
131 This feature is available starting with Perl 5.16.
133 The 'array_base' feature
135 This feature supports the legacy $[ variable. See "$[" in perlvar and
136 arybase. It is on by default but disabled under "use v5.16" (see
137 "IMPLICIT LOADING", below).
138 This feature is available under this name starting with Perl 5.16. In
140 previous versions, it was simply on all the time, and this pragma knew
141 nothing about it.
142 The 'fc' feature
144 "use feature 'fc'" tells the compiler to enable the "fc" function,
145 which implements Unicode casefolding.
146 See "fc" in perlfunc for details.
148 This feature is available from Perl 5.16 onwards.
150 The 'lexical_subs' feature
152 In Perl versions prior to 5.26, this feature enabled declaration of
153 subroutines via "my sub foo", "state sub foo" and "our sub foo" syntax.
154 See "Lexical Subroutines" in perlsub for details.
155 This feature is available from Perl 5.18 onwards. From Perl 5.18 to
157 5.24, it was classed as experimental, and Perl emitted a warning for
158 its usage, except when explicitly disabled:
159 no warnings "experimental::lexical_subs";
161 As of Perl 5.26, use of this feature no longer triggers a warning,
163 though the "experimental::lexical_subs" warning category still exists
164 (for compatibility with code that disables it). In addition, this
165 syntax is not only no longer experimental, but it is enabled for all
166 Perl code, regardless of what feature declarations are in scope.
167 The 'postderef' and 'postderef_qq' features
169 The 'postderef_qq' feature extends the applicability of postfix
170 dereference syntax so that postfix array and scalar dereference are
171 available in double-quotish interpolations. For example, it makes the
172 following two statements equivalent:
173 my $s = "[@{ $h->{a} }]";
175 my $s = "[$h->{a}->@*]";
176 This feature is available from Perl 5.20 onwards. In Perl 5.20 and
178 5.22, it was classed as experimental, and Perl emitted a warning for
179 its usage, except when explicitly disabled:
180 no warnings "experimental::postderef";
182 As of Perl 5.24, use of this feature no longer triggers a warning,
184 though the "experimental::postderef" warning category still exists (for
185 compatibility with code that disables it).
186 The 'postderef' feature was used in Perl 5.20 and Perl 5.22 to enable
188 postfix dereference syntax outside double-quotish interpolations. In
189 those versions, using it triggered the "experimental::postderef"
190 warning in the same way as the 'postderef_qq' feature did. As of Perl
191 5.24, this syntax is not only no longer experimental, but it is enabled
192 for all Perl code, regardless of what feature declarations are in
193 scope.
194 The 'signatures' feature
196 WARNING: This feature is still experimental and the implementation may
197 change in future versions of Perl. For this reason, Perl will warn
198 when you use the feature, unless you have explicitly disabled the
199 warning:
200 no warnings "experimental::signatures";
202 This enables unpacking of subroutine arguments into lexical variables
204 by syntax such as
205 sub foo ($left, $right) {
207 return $left + $right;
208 }
209 See "Signatures" in perlsub for details.
211 This feature is available from Perl 5.20 onwards.
213 The 'refaliasing' feature
215 WARNING: This feature is still experimental and the implementation may
216 change in future versions of Perl. For this reason, Perl will warn
217 when you use the feature, unless you have explicitly disabled the
218 warning:
219 no warnings "experimental::refaliasing";
221 This enables aliasing via assignment to references:
223 \$a = \$b; # $a and $b now point to the same scalar
225 \@a = \@b; # to the same array
226 \%a = \%b;
227 \&a = \&b;
228 foreach \%hash (@array_of_hash_refs) {
229 ...
230 }
231 See "Assigning to References" in perlref for details.
233 This feature is available from Perl 5.22 onwards.
235 The 'bitwise' feature
237 WARNING: This feature is still experimental and the implementation may
238 change in future versions of Perl. For this reason, Perl will warn
239 when you use the feature, unless you have explicitly disabled the
240 warning:
241 no warnings "experimental::bitwise";
243 This makes the four standard bitwise operators ("& | ^ ~") treat their
245 operands consistently as numbers, and introduces four new dotted
246 operators ("&. |. ^. ~.") that treat their operands consistently as
247 strings. The same applies to the assignment variants ("&= |= ^= &.=
248 |.= ^.=").
249 See "Bitwise String Operators" in perlop for details.
251 This feature is available from Perl 5.22 onwards.
253 The 'declared_refs' feature
255 WARNING: This feature is still experimental and the implementation may
256 change in future versions of Perl. For this reason, Perl will warn
257 when you use the feature, unless you have explicitly disabled the
258 warning:
259 no warnings "experimental::declared_refs";
261 This allows a reference to a variable to be declared with "my",
263 "state", our "our", or localized with "local". It is intended mainly
264 for use in conjunction with the "refaliasing" feature. See "Declaring
265 a Reference to a Variable" in perlref for examples.
266 This feature is available from Perl 5.26 onwards.
268
270 It's possible to load multiple features together, using a feature
271 bundle. The name of a feature bundle is prefixed with a colon, to
272 distinguish it from an actual feature.
273 use feature ":5.10";
275 The following feature bundles are available:
277 bundle features included
279 --------- -----------------
280 :default array_base
281 :5.10 say state switch array_base
283 :5.12 say state switch unicode_strings array_base
285 :5.14 say state switch unicode_strings array_base
287 :5.16 say state switch unicode_strings
289 unicode_eval evalbytes current_sub fc
290 :5.18 say state switch unicode_strings
292 unicode_eval evalbytes current_sub fc
293 :5.20 say state switch unicode_strings
295 unicode_eval evalbytes current_sub fc
296 :5.22 say state switch unicode_strings
298 unicode_eval evalbytes current_sub fc
299 :5.24 say state switch unicode_strings
301 unicode_eval evalbytes current_sub fc
302 postderef_qq
303 :5.26 say state switch unicode_strings
305 unicode_eval evalbytes current_sub fc
306 postderef_qq
307 The ":default" bundle represents the feature set that is enabled before
309 any "use feature" or "no feature" declaration.
310 Specifying sub-versions such as the 0 in 5.14.0 in feature bundles has
312 no effect. Feature bundles are guaranteed to be the same for all sub-
313 versions.
314 use feature ":5.14.0"; # same as ":5.14"
316 use feature ":5.14.1"; # same as ":5.14"
317
319 Instead of loading feature bundles by name, it is easier to let Perl do
320 implicit loading of a feature bundle for you.
321 There are two ways to load the "feature" pragma implicitly:
323 · By using the "-E" switch on the Perl command-line instead of "-e".
325 That will enable the feature bundle for that version of Perl in the
326 main compilation unit (that is, the one-liner that follows "-E").
327 · By explicitly requiring a minimum Perl version number for your
329 program, with the "use VERSION" construct. That is,
330 use v5.10.0;
332 will do an implicit
334 no feature ':all';
336 use feature ':5.10';
337 and so on. Note how the trailing sub-version is automatically
339 stripped from the version.
340 But to avoid portability warnings (see "use" in perlfunc), you may
342 prefer:
343 use 5.010;
345 with the same effect.
347 If the required version is older than Perl 5.10, the ":default"
349 feature bundle is automatically loaded instead.
350perl v5.26.3 2019-05-11 feature(3pm)