I might artificially limit the platforms my distribution will install on

Yeah, I like portability myself, and I had the same worry when I started releasing CPAN modules, but experience has shown me that portability is possible. And even if you release a module and it initially fails on a bunch of CPAN Testers, it's possible to bring it back to 100%. As you probably saw, the Shell::Tools tests (which my example code above is based on) pass on Cygwin, Darwin (Mac OS X), FreeBSD, GNU/kFreeBSD, Haiku, Linux, MSWin32, NetBSD, OpenBSD, and Solaris.