I couldn't find any "official" documentation for perllocal and I doubt there is any standard, since naming it so closely to perllocale would be more than unfortunate.

If it's a pseudo-standard by some installers/builders, than the next question is, if such a limited static list in POD is a better choice over searching @INC dynamically.

Hence what's the use-case?

I have some ideas about that, but most of them would be better covered by cpan and cpanm creating something like YAML or JSON.