I don't know exactly what you are doing, but don't forget about modules like Storable for saving db data. I'm using it with a db I'm setting up, and it works very fast. Just put your data into hashes, and Store and Retreive it. The trick is to minmize the disk-read-writes. So you might need to plan ahead so you only read the stored db you need.

A big plus for MySQl or postgresql is that you have multi-user access built-in, although I've read the latest version of BerkeleyDB has "concurrent access".

So I would say the main thing to consider is: "Are you planning to allow simultaneous access", otherwise you should think about file locking.