Does Tie::DBI read the entire recordset into memory? If so, then this might not be a good way to do it for anything other than small tables. I remember once when I took a COBOL class we had to do a comparison like this, what we ended up doing was doing a sort of the two data structures and then a walk through and reconciliation the two sets of records... that may be a better way to do it if Tie::DBI swallows the whole thing into core.