Sorry -- perhaps I wasn't being clear enough: My impression is that CPAN lists standard modules as well as many many more that are not part of the standard Perl library.
When I'm browsing CPAN, and I see a neat module, the first thing I think is: "I wonder if that's part of the standard lib?" That is, I wonder if I've already got it installed (EDIT: also, I'd just like to get to know better which modules are in the std lib, and which aren't.). I sometimes work on various different machines, which each may have different extra modules installed.
That's why I'm curious if the CPAN site offers a way to differentiate (while browsing) between standard modules and other, extra ones. It would seem like a useful web site feature, no?
I realize there's other ways to see: (a) which modules the standard lib consists of, and (b) which other modules I've currently got installed on my system. That said, it still seems that it would be convenient to see directly on CPAN what you're dealing with.