As others have said there is http://rt.cpan.org where you can report bugs.

As you have found few people seem to use the cpan forums. As a module author I get a handful of posts a year on the cpan forums.

What I do pay particular attention to is the cpan testers reports http://www.cpantesters.org/ and via the test reports link on each module on CPAN. One rather easy way to contribute reports whenever you attempt to install a module via cpan shell is to install http://search.cpan.org/~dagolden/CPAN-Reporter-1.1702/ then once it is configured every pass and fail is automatically reported to cpan and the author gets a daily notification of failures.

If tests fail when I am installing a module I first attempt to work out why. Sometimes there are tests which fail because they have not been written to check on module dependencies and often you can either get the dependencies and re-run or if you are confident you don't need that dependency (e.g., if the test needs it but the module you want to install doesn't) you can ignore it.

In summary the nice things to do for module authors would be to report the problem via rt or the forum or via CPAN::Reporter and attempt to fix it. If you can't fix it report it anyway. For yourself, you will either have to get a fix or a least obtain sufficient confidence in the module to decide if you can force the install.