I'm surprised no one has mentioned it so far, but Simon Cozen wrote an awesome tutorial on Perl Internals available here. Soon his book on the subject will be out, and I'm sure that will be even better.

In the meantime, do as much XS (and Inline::C) hacking as you can manage. Almost everything you learn about XS will be useful in the Perl core and the documentation is generally much better.

-sam

PS: See, I can pimp other peoples' books too!