1feature(3pm)           Perl Programmers Reference Guide           feature(3pm)
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NAME

6       feature - Perl pragma to enable new features
7

SYNOPSIS

9           use feature qw(switch say);
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
18           use feature ':5.10'; # loads all features available in perl 5.10
19

DESCRIPTION

21       It is usually impossible to add new syntax to Perl without breaking
22       some existing programs. This pragma provides a way to minimize that
23       risk. New syntactic constructs, or new semantic meanings to older
24       constructs, can be enabled by "use feature 'foo'", and will be parsed
25       only when the appropriate feature pragma is in scope.
26
27   Lexical effect
28       Like other pragmas ("use strict", for example), features have a lexical
29       effect. "use feature qw(foo)" will only make the feature "foo"
30       available from that point to the end of the enclosing block.
31
32           {
33               use feature 'say';
34               say "say is available here";
35           }
36           print "But not here.\n";
37
38   "no feature"
39       Features can also be turned off by using "no feature "foo"". This too
40       has lexical effect.
41
42           use feature 'say';
43           say "say is available here";
44           {
45               no feature 'say';
46               print "But not here.\n";
47           }
48           say "Yet it is here.";
49
50       "no feature" with no features specified will turn off all features.
51
52   The 'switch' feature
53       "use feature 'switch'" tells the compiler to enable the Perl 6
54       given/when construct.
55
56       See "Switch statements" in perlsyn for details.
57
58   The 'say' feature
59       "use feature 'say'" tells the compiler to enable the Perl 6 "say"
60       function.
61
62       See "say" in perlfunc for details.
63
64   the 'state' feature
65       "use feature 'state'" tells the compiler to enable "state" variables.
66
67       See "Persistent Private Variables" in perlsub for details.
68
69   the 'unicode_strings' feature
70       "use feature 'unicode_strings'" tells the compiler to treat all strings
71       outside of "use locale" and "use bytes" as Unicode. It is available
72       starting with Perl 5.11.3.
73
74       See "The "Unicode Bug"" in perlunicode for details.
75

FEATURE BUNDLES

77       It's possible to load a whole slew of features in one go, using a
78       feature bundle. The name of a feature bundle is prefixed with a colon,
79       to distinguish it from an actual feature. At present, the only feature
80       bundle is "use feature ":5.10"" which is equivalent to "use feature
81       qw(switch say state)".
82
83       Specifying sub-versions such as the 0 in 5.10.0 in feature bundles has
84       no effect: feature bundles are guaranteed to be the same for all sub-
85       versions.
86

IMPLICIT LOADING

88       There are two ways to load the "feature" pragma implicitly :
89
90       ·   By using the "-E" switch on the command-line instead of "-e". It
91           enables all available features in the main compilation unit (that
92           is, the one-liner.)
93
94       ·   By requiring explicitly a minimal Perl version number for your
95           program, with the "use VERSION" construct, and when the version is
96           higher than or equal to 5.10.0. That is,
97
98               use 5.10.0;
99
100           will do an implicit
101
102               use feature ':5.10';
103
104           and so on. Note how the trailing sub-version is automatically
105           stripped from the version.
106
107           But to avoid portability warnings (see "use" in perlfunc), you may
108           prefer:
109
110               use 5.010;
111
112           with the same effect.
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116perl v5.12.4                      2011-06-07                      feature(3pm)
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