WHY?

Dist-Zilla is a popular tool for building CPAN distributions. Build process controlled by dist.ini, Dist-Zilla configuretion file. A distribution author can specify license covering his work by a line in dist.ini file:

license = NAME

where NAME is a name of module from Software::License hierarchy.

Software-License is shipped with a set of popular licenses, from Apache_1_1 to Zlib, including GNU General Public licenses (GPL) GPL_1, GPL_2, GPL_3, including their "Lesser" and "Affero" variants (LGPL and AGPL respectively).

So, if a developer wants to release his work under GPLv3 license terms, he should write in his dist.ini:

license = GPL_3

However, Free Software Foundation (FSF) recommends not to stick to a specific version of the GNU license, but keep it up-to-date by using clause "license version N or (at your option) any later version". See the rationalehttps://www.gnu.org/licenses/gpl-faq.html#VersionThreeOrLater. Unfortunately, Software-License distribution does not supply (ready to use out of the box) a way to express such clause.

Software-License-OrLaterPack fulfills the lack, at least partially.