Change the permissions of Build.PL/Makefile.PL to not-executable.
Add a Changelog (best named 'Changes') to the distribution. It should list at least major changes implemented in newer versions.
Add a META.yml to the distribution. Your buildtool should be able to autogenerate it.
Run a proper command ("make manifest" or "./Build manifest", maybe with a force option), or use a distribution builder to generate the MANIFEST. Or update MANIFEST manually.
Fix the version(s).
Add a META.json to the distribution. Your buildtool should be able to autogenerate it.
Add tests or move tests.pl to the t/ directory!
Define the license if you are using in Build.PL. If you are using MakeMaker (Makefile.PL) you should upgrade to ExtUtils::MakeMaker version 6.31.
This is not a critical issue. Currently mainly informative for the CPANTS authors. It might be removed later.