in reply to
What is your favourite Linux or cross-platform database?
I guess I've missed a lot, so what do you expect from a database?
There's only one thing that's important: it should keep my data safe. (This implies having ACID, reliability, etc). Everything else is a secondary feature - although depending on the project, those secondary features could be essential as well.
Features you haven't mentioned: scalability, replicatability, accountability (having the option of turning on auditing), fine grained permission system, vendor support.
Totally irrelevant features for determining whether a database is 'good' or not: its price (your first point); whether it has a good support community; whether there's a Perl interface (that may be important to you, but the existence of such an interface doesn't alter the quality of the database).
As for my favourite database, I'd go with Sybase. But I've only experience with a handful of databases, some of which are unavailable to the general public. And there are many databases I've never worked with. But I've no idea whether my favourite database is suitable for your project. A database for the website of the local chapter of the association of amateur chicken breeders will have different requirements than the database an international telco uses for billing purposes.
Now, tell me again, what does this have to do with Perl?