If it's on BackPAN, it won't be installed. You have to manually fetch it from there, and typically, most users wouldn't be familiar with the process when their 50-distribution project installation fails because something can't be found on the CPAN.
Cleaning up your PAUSE versions just means removing the ones that aren't relatively current. If you remove the dist entirely from the CPAN (all versions essentially), it won't be found using typical CPAN tools period.
ie. Standard CPAN tools don't go looking through BackPAN for previous versions. Those are considered legacy (at least this is my understanding).