I'm running Ubuntu 10.04.
I needed POSIX for a script. Not knowing that it comes with the default install, I opened up CPAN and typed 'install POSIX'. CPAN informed me that it couldn't do so because POSIX is only available for version v5.12.1, and suggested that I install v5.12.1. I had allowed it to auto-accept the defaults, so it went on with downloading and installing while I wasn't looking.