For any brave Phalanx folks who wish to target MakeMaker I can point you at some critically deficient areas of its testing.
1) XS. There is absolutely no testing of XS code. The primary problem is determining if the user has a working build chain. I think Module::Build has code to do this and it might be possible to crib from that. Even just writing the build chain detection code would prove a big help. 2) Common customizations. MakeMaker doesn't have much of an API for altering the Makefile so users are forced to do s/// over the make code to do anything interesting. Apparently innocent internal formatting changes and renaming of internal targets can often break these customizations. The more common customizations need to be identified and tests for them written. Modules to look at include Mail::SpamAssassin, PDL, mod_perl, Tk, WxPerl, Archive::Zip and Apache::PageKit (IIRC). Even if you don't actually write the tests, just a catalogue of common customizations, possibly posted on the MakeMaker wiki, would prove useful. Consult the PATCHING document for some information as to MakeMaker's, umm, special cross platform considerations.
