#!/usr/bin/perl
use strict;
use warnings;

my %hash = ( a => 1, b => undef);

foreach my $key ('a', 'b', 'c') {
        print "$key " . (defined($hash{$key})?"is":"is not") . " defined\n";
        print "$key " . (exists($hash{$key})?"does":"does not") . " exist\n";
}

##</code><code>##

a is defined
a does exist
b is not defined
b does exist
c is not defined
c does not exist