I know I can use perlcritic as "compiler" - that's how I use it with syntastic. :)

As for flymake, I don't think it's doing anything interesting. If you check the code, you'll see it start on file save, just like all other syntax checkers for Vim, and just run external checker using usual blocking system(), not in background. So, flymake is just basic version of much more advanced syntastic plugin.