in reply to Perl modules and the GPL
Nothing in its Makefile.PL, or its README or anywhere that I can see would allow me to re-use Sort::Topological.
FYI: Graph is another module which also implements topological sort. And that module is licensed under the same terms as Perl itself — which I interpret as "if you can use Perl, then you can use this module".
I agree that this kind of legal questions are a rather silly waste of time... I'm pretty sure no CPAN author intended his module to pose this kind of legal problem. It's just that they probablby slapped any convenient type of license on the module when they first released it.