1lexical::underscore(3)User Contributed Perl Documentationlexical::underscore(3)
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NAME

6       lexical::underscore - access your caller's lexical underscore
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SYNOPSIS

9          use 5.010;
10          use lexical::underscore;
11          use Test::More;
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13          sub is_uppercase {
14             my $var = @_ ? shift : ${lexical::underscore()};
15             return $var eq uc($var);
16          }
17
18          my $thing = 'FOO';
19          my $works = 0;
20
21          given ( $thing ) {
22             when ( is_uppercase ) { $works++ }
23          }
24
25          ok($works);
26          done_testing();
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DESCRIPTION

29       Starting with Perl 5.10, it is possible to create a lexical version of
30       the Perl default variable $_. Certain Perl constructs like the "given"
31       keyword automatically use a lexical $_ rather than the global $_.
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33       It is occasionallly useful for a sub to be able to access its caller's
34       $_ variable regardless of whether it was lexical or not. The "(_)" sub
35       prototype is the official way to do so, however there are sometimes
36       disadvantages to this; in particular it can only appear as the final
37       required argument in a prototype, and there is no way of the sub
38       differentiating between an explicitly passed argument and $_.
39
40       This caused me problems with Scalar::Does, because I wanted to enable
41       the "does" function to be called as either:
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43          does($thing, $role);
44          does($role);  # assumes $thing = $_
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46       With "_" in the prototype, $_ was passed to the function at the end of
47       its argument list; effectively "does($role, $thing)", making it
48       impossible to tell which argument was the role.
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50       Enter "lexical::underscore" which allows you to access your caller's
51       lexical $_ variable as easily as:
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53          ${lexical::underscore()}
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55       You can access lexical $_ further up the call stack using:
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57          ${lexical::underscore($level)}
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59       If you happen to ask for $_ at a level where no lexical $_ is
60       available, you get the global $_ instead.
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62       This module does work on Perl 5.8 but as there is no lexical $_, always
63       returns the global $_.
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65   Technical Details
66       The "lexical::underscore" function returns a scalar reference to either
67       a lexical $_ variable somewhere up the call stack (using PadWalker
68       magic), or to the global $_ if there was no lexical version.
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70       Wrapping "lexical::underscore" in "${ ... }" dereferences the scalar
71       reference, allowing you to access (and even assign to) it.
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BUGS

74       Please report any bugs to
75       <http://rt.cpan.org/Dist/Display.html?Queue=lexical-underscore>.
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SEE ALSO

78       PadWalker.
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AUTHOR

81       Toby Inkster <tobyink@cpan.org>.
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84       This software is copyright (c) 2012, 2014 by Toby Inkster.
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86       This is free software; you can redistribute it and/or modify it under
87       the same terms as the Perl 5 programming language system itself.
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DISCLAIMER OF WARRANTIES

90       THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
91       WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
92       MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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96perl v5.28.0                      2017-01-30            lexical::underscore(3)
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