You don't have to convince him; you just have to get him to change his behavior. Isn't there a quality rating system for CPAN modules? I don't know how thorough or how effective it is, but if "build warnings" are considered unacceptable, that may get his attention.

Well, it is possible to write a review against a module and rate it. However, I have seen that venue become a vehicle for retaliatory activities.

Then there is CPANTS. However, they don't build modules, leaving that activity to CPAN testers.

As to CPAN testers, there is no criteria related to build warnings - just 'make test' results.

So AFAIK, there is no feedback mechanism for build warnings other than reporting them to the developer (as I did, along with a patch) with RT being the most effective method for that purpose.