Getting Involved: Revision 5
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!
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.
Also, scan through the Synopsis which is cross linked with the test suite. Look for sections of the spec that don't mention test scripts. These means there is a either a test script without a smart link, or no tests have been written. (t/TASKS explains about smart links and much more!)
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.