Well, you'll probably be using DBI, so it takes care of all the different 'interactions.' (DBI is a generic database tool that handles all the complex stuff between your code and the DB ... search for it on CPAN).

MySQL and PostgreSQL are Open Source databases (so, they're free). MySQL is fast and PostgreSQL is powerful. Most folks that I've talked/chatted/emailed recommend PostgreSQL.

I'd stay away from msSQL, Access and Oracle becasue of the price. PostgreSQL should do what you need it to.

Check out this link: http://www.commandprompt.com/ppbook/

It's the O'Reilly PostgreSQL book online (it's a legal copy by the authors ... part of the Open Publication License).

Hope this helps!