1. Your repositories belong to you, the naming is up to you. A perl- or p5- prefix sounds more like the sort of thing I'd expect for OS packages.

A typical place to see repository names is at the landing page for an author (like mine) or the repository list (like repositories); as you will see from those examples, I tend to prefer short repository names, but try to augment them with an at least somewhat useful description.

When you go to a specific repository, github will try to find a top-level README (plain text) or README.md ("github markdown") to show, as at krun. This is where you can go much further to help a casual browser understand whether this is the repository they are looking for.

2. I don't know of any such connection, but I'm sure enough people are now using git as the revision control system for their CPAN code that someone (several someones) will have automated the process of releasing to CPAN from git. In that respect, github would likely be no more than "just another git repository" - the automation would most likely use git commands directly.

3. I never saw any reason to have multiple github accounts myself. I can't think of any technical reason to want them, but there are possible social reasons: perhaps because you work for a company that likes to claim copyright in things employees do in their own time; or because you feel the type of code you write in your own time risks reflecting badly on you professionally.