#!/usr/bin/env perl -l

use strict;
use warnings;

my %test_hash;

if ($test_hash{unset_key} =~ /x/) {
    print 'true';
}
else {
    print 'false';
}

unless ($test_hash{unset_key} =~ /x/) {
    print 'false';
}
else {
    print 'true';
}

##</code><code>##

Use of uninitialized value $test_hash{"unset_key"} in pattern match (m//) at ./pm_example.pl line 8.
false
Use of uninitialized value $test_hash{"unset_key"} in pattern match (m//) at ./pm_example.pl line 15.
false