If I understand correctly, you're looking for all uses of _ as a name space. You are trying to do this by changing the way Perl parses the module you are working on. Of course, you could do this (at least in theory), but I am in no position to advise you on how, and you might want to contemplate the module quote at the beginning of toke.c before you go too far down this road.

Have you considered other routes to your goal? Maybe a source analysis using PPI and/or App-Ack would serve you better. Be warned that the App-Ack install may fail the first time you try it if you don't already have all the prerequisites installed. The solution, of course, is just to rerun the install.