On my perl-5.10 I had Test-Harness-2.64. I updated to version 3.10, and Math-Pari still builds and tests flawlessly for me. Maybe you ought to switch to Win32 :-)))
Sorry, I don't know why 'make test' reports that absurd "FAIL" for you. It may signify a problem with Math::Pari ... or it may signify a problem with something else.