From your question it does not look like you are evaluating Perl approach vs SQL. But anyways here is my take on redundancy - If you want Perl approach, I would use a Hash as tanktalus has mentioned above. This assumes your data is not HUGE! If so then you want keep the information indexed/sorted so that you can check for the key very quickly (also mentioned above).

If you are looking for SQL way then you should consider setting that field as UNIQUE. If you try to update a field that will vioate this unique constraint MYSQL will throw a warning and not update (assuming you will not be using the IGNORE option). If you are updating the data from Perl then you should be able to trap the warning/failure to execute query and report back to the user.  show warnings in MYSQL should give the warning that was generated.

cheers

SK