$legacy_data { unique field value across both records } = {
  		'goodField' => 'I am good!',
		'firstField' => 1,
		'secondField' => 3
	};
$new_data { unique field value across both records } = {
		'firstField' => 11,
		'secondField' => 33,
		'goodField' => 'I am good!'
};
$differences = [ firstField, secondField];

##</code><code>##

foreach my $record_id (keys %legacy_data) {
  my $legacy_data = $legacy_data{$record_id};
  my $new_data = $new_data{$record_id};
  ....
  # here go all the tests you said
  ....
}