Add a section called "LICENSE" to the documentation, or add a file named LICENSE to the distribution.
Add =head1 LICENSE and the text of the license to the main module in your code.
Add a META.yml to the distribution. Your buildtool should be able to autogenerate it.
Add =head1 LICENSE and/or the proper text of the well-known license to the main module in your code.
Add a META.json to the distribution. Your buildtool should be able to autogenerate it.
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.