First, this wouldn't be possible at all, as there is no
central all-controling CPAN site. There are many mirrors,
and not all mirrors are known. Furthermore, people put CPAN
on CDROMs, and distribute them.
Second, download figures don't mean anything. I may download
two modules, ditch one of them because it doesn't even want
to build, and just use the second. But I still have downloaded
both modules the same amount of times.
Third, it would actually "favour" buggy, unstable, modules.
They (hopefully) get new releases, and hence, new downloads,
because people hope the bugs get fixed. Stable modules don't
get updated often, so there's less need for an upgrade (and
even if there's a new release, there's often no need to bother
with an upgrade). So, they would get downloaded less often.
Abigail