|Don't ask to ask, just ask|
Hi, I'm the guy in charge of maintaining the UBB source.
No, really, I am. I have a login here, but I'm not using it to post this, as I've asked some potentially embarassing questions and don't want blackmail material. ;)
If you are going to try and work with the UBB code, by all means, grab the current beta version, 6.1.0, release 2... it's far better than the 5 series that the guys below describe.
We use CGI.pm (our own modified version, at least - stupid web hosting providers!), it runs under strict, and will eventually run under warnings.. ;)
The original author of the script threw the entire thing out and rewrote it for 6.0... but he made some poorly planned design decisions and it turned out to be less than spectacular.
We're finishing up over five months of reorganizations and optimizations, including a completely new I/O library and a whole bunch of other interesting stuff.
See, you can actually WORK with it now. ;)
Others have posted links to UBBDev - head over there, it's a great resource. ;)