I'd like to see something on XML... and while that's hardly a topic restricted to Perl, using Perl tools to implement an XML solution is surely a viable topic.
Actually... and this might fall outside a strictly Perl domain... what I'd really like to see are case studies; here's the problem, here's why XML was used, here's what we did to reach our solution (Perl-y bits here), here's the solution. There's plenty of books, tutorials, etc on 'What is XML?', but bloody little on how/why you implement it.
Or I'm being dense. :)