Unfortunately, the honest answer is "it depends."
Sometimes, there is a section at the bottom of the module docs that explains this, but not for IO::Socket::Multicast.
Failing that, you can try looking up the module in search.cpan.org. When I do this, I get http://search.cpan.org/~bramble/IO-Socket-Multicast-1.11/. That page, in turn, generally has a link on where to submit bugs. In this case, it points to rt.cpan.org. So one thing to try is to submit your patch there. A look says that there are some _old_ open bugs, but that the latest submission got a response from both Andrew Bramble and Adam Kennedy.