perl - 给定/当值未定义时

Question

在下面的代码中，我得到一个 uninitialized value警告，但仅限于第二个 given/when例子。为什么是这样？

#!/usr/bin/env perl
use warnings;
use 5.12.0;

my $aw;

given ( $aw ) {
    when ( 'string' ) { 
        say "string"; 
    }
    when ( not defined ) { 
        say "aw not defined"; 
    }
    default { 
        say "something wrong"; 
    }
}

given ( $aw ) {
    when ( /^\w+$/ ) { 
        say "word: $aw"; 
    }
    when ( not defined ) { 
        say "aw not defined";
    }
    default { 
        say "something wrong";
    }
}

我得到的输出是:

aw not defined
Use of uninitialized value $_ in pattern match (m//) at ./perl.pl line 20.
aw not defined

Answer 1

given / when 使用“smartmatch operator”:~~ .
undef ~~ string是:

undef     Any        check whether undefined
                     like: !defined(Any)

因此这里没有警告。
undef ~~ regex是:

 Any       Regexp     pattern match                                     
                      like: Any =~ /Regexp/

尝试匹配 undef 时会产生警告.