There exist quite good Perl forum named gForum. It does play nicely with mod_perl, supports SQL backend, is easily extendable with plugins written in Perl and its source code doesn't suck too much (I had to integrate its authorization with customer website so I've spent some time reading gForum sources and writting plugin for it).

Disclamer: of course I don't work for them :).

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Ilya Martynov (http://martynov.org/)