I think both of the previous responses to your question make a lot of sense, possibly more sense than my suggestions. But I'm going to throw them out there anyway :)

Two more ways to get into hacking perl guts:

1. Start with XS
I first gained familiarity with perl's guts by getting some C code working in perl using the Swig tool, which was unfortunately not quite adequate for my needs at the time. Gluing some additional code into perl via XS or Swig extensions can introduce you to some of the basics of the perl guts, without drowning you. It also lets you compile additional useful functionality onto perl, instead of modifying something which mostly works to begin with.

2. Go directly to perl 6
In the long run, perl6 guts knowledge may be more useful than perl5 guts knowledge. If you don't have a particular need to know perl5 in the near future, you might want to volunteer some time to help make perl 6 a reality.

Or maybe not, who knows :)

Thanks,

Alan