The only problem with using this as a metric is that it doesn't measure whether or not you ever used it after you installed it, or what your opinion of it was. I've downloaded *tons* or modules from CPAN, but that doesn't mean that I would rate them all the same. Sometimes I'll download one thinking it may do what I need, but upon closer examination I find that it doesn't (or worse, doesn't even work). I wouldn't want my constant useage of HTML::Template to be rated the same as my curiosity download of (the previously mentioned) Natalie::Portman.
I don't think counting downloads alone in any fashion is going to provide the kind of info to be really helpful. However, it would be good to have it along side a 'vote' based system, so that you could tell what percentage of the people that downloaded it rated it well.
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"