What's the best "free" DB for a web-based app?

[John Armstrong]

Hi - I'm soon to be doing a Perl app on the Internet, that'll need
database. We want the db to be as "free" as possible, but still fully
multi-user (web-based). Would the best route be MySql on Linux? Random
access files? Something else? We want no licensing obligations (no Oracle,
Sequal Server, etc.). We want to go with Perl because it's the best
programming language invented by humans.