You could do this which eliminates the need for executing separate append operations for each read:

1. Open an ftp connection to the remote server
2. Initiate an append operation and open the data connection to the ftp server.
3. Start your read loop and write what you read to the data connection. Also perform whatver tracking of the data here.
4. When done reading, close the data connection. (Crossing your fingers before closing the connection would be advisable here.)

Basically you would be customizing the _store_cmd() routine in Net::FTP with your own process. Sub-classing Net::FTP actually might not be a half-bad way to do this.

Update: Adding a call-back to Net::FTP for progress tracking shouldn't be too hard. It already prints out hash marks using this code in _store_cmd():

    if ($hashh) {
      $count += $len;
      print $hashh "#" x (int($count / $hashb));
      $count %= $hashb;
    }
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Just replace that with your own code.