That matrix only covers platforms that CPAN-testers have used. CPAN-testers have pretty good coverage on Unix-a-likes and Windows, but there's no-one testing regularly with VMS, RISC OS or Amiga. And those are the sort of platforms that you'd expect to have the most portability problems.

I have anecdotal evidence that most modules don't pass their tests on VMS, and that VMS users are used to patching them locally. Unfortunately, I don't think any of them have got round to patching the CPAN-testing tools and submitting patches back to the maintainers. Anyone with a home VAX who can dedicate some time to that? Maybe you could apply for a TPF grant!