Cpan-recent-growler.pl is a project mainly written in PERL and SHELL, it's free.
cpan-recent-growler
Fetches CPAN::Recent::Upload and growls as new activies are coming in.
There's a binary build for Mac OS X available at Downloads.
By default this scripts fetches the CPAN updates every 300 seconds and displays at most 10 Growl notification per fetch and you can change the both settings with Mac OS X preferences:
defaults write net.bulknews.CPANRecentGrowler initial_interval 21600
defaults write net.bulknews.CPANRecentGrowler interval 180
defaults write net.bulknews.CPANRecentGrowler maxGrowls 5