in reply to Hacking Perl (I think)
Reading perlguts, PerlGuts Illustrated, perlxstut and perlxs might help. The Porting directory in the perl source also contains a lot useful info.
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...
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.
The p5p mailing list (subscribe or archive) and #p5p on irc.perl.org could be sources of assistance, as well as here.
At some point, you will have to learn how to use git. There's a free book, or a more perl oriented cheat sheet in perlgit.
Update:
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...
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