Volunteers with lots of different perl versions and OSes run software that monitors CPAN for new uploads, and then automatically downloads them and runs their automated tests, and uploads the results.

Here's more on CPANTesters: http://wiki.cpantesters.org/wiki/WhatIsCPANTesters

And here's an example of the results for a popular module: http://matrix.cpantesters.org/?dist=Moose+2.1600