Typically, what has happened is that you have started cpan, and then used ^Z to put it to sleep, or changed to a different terminal. Then you try to start it again. It checks for a lock file, finds one, and complains.

It is not a serious issue. Unless you happen to be running a lengthy install in the other CPAN process, there shouldn't be any conflicts.

I've never had a problem overwriting the lock file.

Clint