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

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