I remember looking at the Ruby source code about a year ago, so it may have drastically improved since then, but I remember it being a bit of a mess. But it is quite fun to step through the code in gdb and you can learn quite a lot by hacking on it or implementing a DLL extension.
If trolling through language implementations is your kind of thing, I'd suggest having a look at Lua or Python as well (I never got Perl to configure, let alone compile) - then you can compare and contrast with the way things are done in Ruby.
Let me second that. A few months ago I was looking at Ruby, Python, and MzScheme's GCs, and I found that Ruby code was hardest to read and badly commented. Python, on the other hand, was very nicely written and commented.
Lua too, but Lua also has the advantage of being so tiny it's easier to read.
If trolling through language implementations is your kind of thing, I'd suggest having a look at Lua or Python as well (I never got Perl to configure, let alone compile) - then you can compare and contrast with the way things are done in Ruby.