Oh well. For almost anything that you can easily do with flat files, I'd strongly recommend them over an RDBMS anyway.

Once pure flat files become limiting, using simple tied interfaces to files will buy you a lot of simplicity and give you access to structure. Especially, the most useful recommendation that I picked up from merlyn, which is DBM::Deep.

By the time your application find the limitations of that (other than size), you'll probably be in need of a proper RDBMS anyway--but that's usually a way off at the beginning.