I'm a fan of wikis in general, however in my experience documentation wikis are very hit or miss. A while back I was learning Rails for a side job. There were a few nice tutorials and the API docs were very thorough, but developer (internal not application) oriented. On the other hand the Rails wiki at the time was not very useful. It covered all the basics, but it had gaps in many of the intermediate areas of Rails (from my point of view). It may be that my problems were unique, or that my general lack of experience in Rails (and Ruby) exacerbated my difficulties. In the end I felt like the Rails developers hadn't written good user oriented docs and abdicated that responsibility to the community.
I have certainly seen worse documentation on CPAN, but never for a project that large or popular. A wiki may improve a projects documentation, but IMO it needs a strong developer written base to start from. Promoting wiki as the official Perl document source isn't likely to encourage developers to be thorough in their documentation. It's just one more excuse to do the documentation later.