Hopefully you can provide more info about your application, or you already know which database is best given that it works with Perl..

One note I should make about Mysql and PostgreSql. Yahoo runs on Mysql; it is not a toy database. Some people think it is a "toy" because it was not originally ACID compliant, however 1) it is a very cost-effective way to do lots of reads, and 2) there are now one or more table types which handle transactions. PostgreSql is (for me) harder to use however it feels like a cheap Oracle, meant in a good sense; it has very nice advanced features and it has in the past year or so become much more stable and much faster. Mysql and PostgreSql are in a war about stats and also the code of each is continually improving, for example Mysql's transaction support and also replication is new.

They have different philosophies and may be better for you depending on your needs. But which free db you pick doesn't really matter as far as Perl is concerned. Many people use Mysql or PostreSql it seems. Unless you already know that you must use Oracle, I'd stay away from it since there is quite a lot of learning and expense involved. MS SQL seems quite nice if you are a Windows-only house, however I am very unimpressed with the way MS dealt with a bug in it originally (the one that lost nuclear data), and with Microsoft in general. I'm not saying they're evil.. but I think if you happened to come across some kind of a bug you would get it fixed more quickly with some other group, or you might be able to fix it yourself or hire someone to do so, since you have the source code.

Finally, if you stay away from using db-specific features, you can use DBI (or maybe DBIx) to abstract data from the database you use. Theoretically this means you can switch databases painlessly. All I know is I once switched from MySql to Oracle and it wasn't painless! ;)

Anyway, databases are a big subject but I probably would recommend keeping things simple so that you can change databases some time if you like, and (assuming you are not a db professional) start with something that combines a lot of power with little effort like Mysql. Or PostgreSql if you want, many programmers find great satisfaction and fulfillment in it. Good luck!