I meant: how can I browse CPAN itself to see a listing of all the distributions there, rather than just looking at the listing of modules.

The index file modules/02packages.details.txt on CPAN contains a mapping of all modules to distribution names.

My guess is that most packages on CPAN are single-module packages, and that there far fewer multi-module "distributions".

That's an interesting question. Of the 12,166 distributions listed today in 02packages.details.txt, 8592 have only one module associated with them. However, some of these single-module distributions are older versions of distributions whose current version contains multiple modules (eg, look at Slauth-0.01.tar.gz and Slauth-0.02.tar.gz). This sometimes means that a module in an older distribution has been replaced, or perhaps been made obsolete, by another module in a newer distribution, but it still shows up in 02packages.details.txt because it's still registered with PAUSE.