Overall I think your post is very good. It answers most of the OP's questions, and doles out good advice. I do have a nit to pick with your opening sentence, however. I have seen this claim made a number of times through the ages, and it always struck me as odd. It just didn't feel right. So I decided to do some research....

Based on what I could find, the native POSIX subsystem in Windows NT/2000 is limited at best. It seems to be incomplete, and even bordering on non-functional. According to a paper on Microsoft's own MSDN, "the Windows NT/2000 POSIX subsystem ... only supports POSIX 1003.1," and "The 1003.1 system is of limited interest for fully featured applications, because it does not include many capabilities (such as those in 1003.2, network support, and so on)." You can read the entire article to get full context.

I guess the claim that Windows NT/2000 has a POSIX layer is true, but it seems rather disingenuous to say "Windows NT is a POSIX compliant OS." How this relates to porting of Perl apps to Windows, I'm not sure. It just seems like something that should be pointed out for future reference.