Have you actually compared the rate of your script's file transfers to file transfers using ftp from the command line? Regardless of the size of your local pipe (it's all pipes isn't it?), the rate of the transfer may be limited by a high load on the remote server or bandwidth constraints somewhere upstream.

Having said that, if the local and remote machines use the same type of line ending, you may see some speed-up from using binary mode (i.e. no transformation of the data being transmitted).

$ftp->binary()
  or die "Cannot change to binary mode";
[download]

Perhaps some other monk has some insight into whether futzing with the block size would make any difference.