From the Net::FTP docs:

Put() Returns REMOTE_FILE, or the generated remote filename if REMOTE_FILE is not given.

So, what you are getting is NOT an error.

The code corresponds to the FTP Server status 250:

250 Requested file action okay, completed.

I'm not sure why the message() does not correspond to the code() - perhaps the programmer makes it correspond only in error cases .. that would be a minor bug, to me.