Reading perlguts, PerlGuts Illustrated, perlxstut and perlxs might help. The Porting directory in the perl source also contains a lot useful info.
However, there's a lot of source code with much magic hidden within. I'd suggest a more focussed approach, such as picking a TODO from perltodo and work towards DOing it.
Another avenue you might like to try, is looking at perl5's bug-tracker (it's much easier to use if you log in first) and see if there are any bugs you can try to fix.
Update^2: Corrected address of #p5p (thanks moritz!)
Perl: perlintro perlretut perldoc Basic debugging checklist Yes, even you can use CPAN
PerlMonks: How do I post a question effectively? Markup in the Monastery How do I post a question effectively? I know what I mean. Why don't you?
General: How To Ask Questions The Smart Way What every computer scientist should know about floating-point arithmetic
To Be Continued...