given/when with undefined value

Question

In the following code, I get an uninitialized value warning, but only in the second given/when example. Why is this?

#!/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";
    }
}

The output I get is:

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

Answer 1

given/when uses the "smartmatch operator": ~~.

undef ~~ string is:

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

Thus there is no warning here.

undef ~~ regex is:

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

And a warning is produced when trying to match on undef.