sudo apt-get install libcrypt-ssleay-perl
Of course if you are not running Ubuntu then you will need to use CPAN or find whatever repackaged garbage that your distro is using to distribute this ;-).
One other cause could be that your root certificates are outdated, so if you have the aforementioned PM installed and are still receiving a 501.. this is likely the cause... google how to update your root certificates for your distro! Again, for the sake of completeness, this is how you do it on Ubuntu:
sudo apt-get install ca-certificates
I have also added this to the PP FAQ.