my $hash_ref = {};
unless ( defined $hash_ref->{'some_key'}) {
    print "some_key is not defined\n";
}

my $other_hash_ref = { 'some_key' => 0 };
if ( ! defined $other_hash_ref->{'some_key'}) {
    print "some_key is not defined\n";
} else {
    print "some_key is defined\n";
}