After I posted the above note (License of Perl Modules on CPAN) on the module-authors mailing list I got a quick reply from David Landgren telling me that it has been there for several years now.
Wow. If that's true, and I checked in 6.31, the oldest version of MakeMaker currently available of CPAN and it was already in the docs back in October 2006, then why are there still so many modules on CPAN without a proper license field in their META.yml file?
CPANTS says there are currently 9920 distributions without a license field in the META.yml file. (Out of the 13222 or so distributions.)
Published on 2008-03-25 by Gabor Szabo