dont sweat it, this is easy (ish) ;)

Youll find plenty of DBI examples online. Youll find everything you need for integrating with phpBB on the guys server, ie in the .php files.
I know the inside of phpBB quite well* - its pretty easy to follow.

So, grab some coffee and start looking through the php files. Youll see how phpBB creates sessions, what SQL statements it uses, how it checks cookies and what it looks for etc
just translate the bits you need back into your own app using DBI and perl.

DBI is probably the greatest Perl module, you want to get to know it better anyway. This could be a really good learning experience for you.

*when i first started learning perl i rewrote some of phpBB in perl as it covered a lot of important web-dev concepts and like your friend, I wanted to integrate stuff round the solid forum database. I might still have a few of those files on my HD. If i find anything worth repeating Ill message you