Can you create a very short script that shows the problem? Maybe it's something like old handles aren't being closed properly, so the FTP server sees you as having 4 simultaneous connections.

Also, you can turn on debugging for Net::FTP to see exactly what's going on. That might give a useful clue.

One more idea: As a workaround, you could try my module Net::FTP::AutoReconnect, which will automatically re-establish your FTP connection if it fails for some reason. I created it to deal with some quirky, unpredictable FTP servers, and it might help in your case.

Good luck!