(Supposing it's not your own.)
Personally, I think one should take the following steps:
This is really where even most experienced people seem to stop. Unfortunately, that is often not enough. Not all module authors use RT. Some deliberately ignore it, some don't like it, some don't know or understand it. Some have disappeared altogether. If one filed a bug against an actively-maintained module and there was evidence that the author used RT, this would be enough. Otherwise:
Now, you might say that you don't have the time to do this. Nobody really does. But if you had the time to write a coherent bug report on RT, it would be a waste of your previous effort not to make sure the module actually gets fixed.
Getting in touch with authors can be tedious and time consuming, but most are willing but busy and gladly accept help. If you have a fix, consider donating a couple more minutes to apply it and roll a new distribution.
CPAN is Perl's greatest strength and the abundance and quality of software on CPAN is our best selling point. Let's keep it that way.
Thanks for reading!
Update: Now mentioning that it's a good idea to check for the author's preferences concerning bug reports.