show the difference between defined and exists

testdefined.pl

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my %hash = (
    id  => undef,
    id2 => 2,
    );

if (!$hash{id}) {
    say "'id' is not defined...";
}
if (exists $hash{id}) {
    say "but it exists in the hash!";
}

if (exists $hash{id2} && defined $hash{id2}){
    say "'id2' exists and is defined.";
}

## running this prints:

# 'id' is not defined...
# but it exists in the hash!
# 'id2' exists and is defined.