You need to drop XML like a hot rock if speed is a primary concern and you control both sides of the transaction. XML isn't optimized for speed, it's optimized for readability and extensibility. Even if you cheat by writing your own regexes you'll still have to contend with the overhead added by all the "<foo></foo>" repetition.

Have you considered Storable? As far as serializing Perl data-structures goes it's the undisputed speed king. Depending on your access pattern you might also consider DB_File. When used correctly it can be quite fast.

-sam