The added benefit comes when you want to port a project you wrote in one language to another language.
The possibility of interested people porting from Perl 5 to Rakudo (aka Perl 6) suggests that saying p5 is better than saying perl or cpan somewhere in the name. (Plus p5 is short. Short is good.) The fact that p5-... looks similar to existing naming conventions suggests to me that this is a good way to name things.
Whether we should get all official and call it a "standard" is another thing entirely. But I'm definitely OK with calling it a "suggestion".
Update: I left out a "p5".