What does the cpan testers wiki say?

IIRC Devel::AssertOS

update: http://wiki.cpantesters.org/wiki/CPANAuthorNotes "How can I indicate that my distribution only works on a particular operating system?"

# in a Makefile.PL or Build.PL
use lib 'inc';
use Devel::AssertOS qw(Linux FreeBSD Cygwin);
[download]

I do want to say, I've seen once or twice these types of tests used because of a dependence on /bin/gzip or some typical unix program ... they do exist on windows also :) so test for the program if thats the real dependency