Getting Involved: Revision 9
Hacking on Perl 6 code
Reading Material for Pugs Hackers
Besides these links, the Pugs distribution also provides README, READTHEM and READTOO.
General Perl 6 Tasks
A running list of tasks to work in kept in the repo at TASKS. It's a catch-all list. Something on it is bound to match your skill set!
A specific test to help with is Smart Linking.
Perl 6 Tasks for Perl 6 programmers
Preludification Candidates discusses parts of Perl 6 that can be written in Perl 6.
There's also plenty to do related to the test suite, which is written in Perl 6. See t/TASKS for work to do there.
You might also be interested to Start Your Own Perl 6 Module, or work one of the existing ones.
Hacking the Perl 6 docs
In general, offical docs are stored in docs/Perl6 and docs/Pugs in POD format, and they are fair game to improve directly. The exception is most of the documents in docs/Perl6/Spec which is the official Perl 6 specification. Improvements to the official spec should be suggested to the perl6-language list.