Read all the available documentation: perlguts, perlhack, Gisle Aas' Illustrated PerlGuts, Simon Cozens on modifying the optree, Simon Cozens on perl internals, My own list of interesting links. Get used to using Devel::Peek, read the header files in the source code, heck - read relevant parts of the source code. Even better - create a private build of perl with symbols so you can step through a perl program using the gdb debugger.

Remember to have fun.