Collaboration is really just highly focused content management.

So the Open source CMS contenders I am aware of are.

Everything Engine http://www.everydevel.com/
Slashcode www.slashdot.org
Callisto CMS www.callistocms.org
Live Journal http://www.livejournal.com/
ZOPE www.zope.org
PHP Nuke http://phpnuke.org/
PHP Website http://phpwebsite.appstate.edu/

these links may also help
www.cms-list.org
www.la-grange.net/cms

For the Perl ones mod perl is a must. The big question is do you let your users type in

HTML

or use some other custom mark up language