I'm getting this too under 5.8 on MDK 9. It appears that it's not possible to set the local port from which Net::FTP connects, which means that unless there's some smart code in the XS module underneath IO::Socket choosing a local port depending on the threading environment, it's just using the same port. Supposedly you can make that non-blocking by setting the "Timeout" argument when you create your Net::FTP, so try giving that a shot.