So, what does it actually do? After a few minutes of discussions with peers I've come to the tentative conclusion that it allows you to build CPAN-like module collections, including with custom versions.

Is that right?

In any case you should improve the wording. custom repositories of Perl modules is rather vague. My first thought was "github also allows me to host custom repositories, and sure I can put Perl modules in". In particular "repository" is a rather broad term; maybe you can find some more specific term?