Add a LICENSE file to your distribution - it's easy!
Every distribution should have a LICENSE file, that corresponds to the licensing information contained in your Makefile.PL.
You can create this file from the command line by installing App::Software::License - e.g. cpanm App::Software::License. Then, just invoke the software-license command.
e.g. for the DBI distribution, using the year of the first release (from the Changes file), the main author's name (from Makefile.PL) and license type (also from Makefile.PL):
software-license --holder 'Tim Bunce' --license Perl_5 --type fulltext --year 1992 > LICENSE
Then, commit the new file to your revision control system, and add the new file to MANIFEST (either manually, or by running make manifest).
If you are using Dist::Zilla to manage your distribution packaging, you can skip all this, and simply add [License] to your dist.ini. (It's also in the [@Basic] plugin bundle.)
Toolchain cog, Moose release manager, CPAN curator.
Potential users please note - the library underlying the software-license executable is not as specific as it could be. See RT#105867 for more details.
I am not outright saying "do not use App::Software::License", only "examine the output and see if it is sufficient for your needs"