The easiest way is just to use a transparent proxy server (written in Perl), which dumps out all traffic going through it. Of course, this will not work if the browser behaviour differs wildly from the no-proxy behaviour.

httptech had a small proxy in RE: Benevolent Ad Filter, and in that thread there is also mentioned FilterProxy.

It shouldn't be too hard to glue together some frankensteinian abomination with Net::Daemon, Net::Server and LWP either, but that means your platform needs fork() support I guess.