If you look at the files generated

The eumm generated MYMETA.json/META.json use meta-spec 2.0 http://search.cpan.org/perldoc?CPAN::Meta::Spec where license is an array

while the MYMETA.yml/META.yml uses http://module-build.sourceforge.net/META-spec-v1.4.html#license where license is a string

I don't see how ExtUtils::MakeMaker, which only allows a single license value, could create the multiple license values as described in CPAN::Meta::Spec.

Its possible using https://perldoc.perl.org/5.30.0/ExtUtils/MakeMaker.html#META_ADD/META_MERGE

The yml files end up with a string, the json files end up with an array

LICENSE

Available in version 6.31 and above.

The licensing terms of your distribution. Generally it's "perl_5" for 
+the same license as Perl itself.

See CPAN::Meta::Spec for the list of options.

Defaults to "unknown".
[download]

This means it takes a string