If you're really serious about building middle-tier OO systems on top of an RDBMS, I recommend reading the manuscript for Martin Fowler's next book, Patterns of Enterprise Application Architecture. (The manuscript will be on-line until the book is published later this year.)

The examples in the book are in Java or C#, but translate fairly well to Perl.

The short version is there are choice you need to make about locking models. These choices affect how you manage caches, and how you ensure that the right changes get written back to the database in the right order. Getting a middle-tier system working well requires a lot of careful bookkeeping.