I've worked out the problem, in case anyone's interested - instead of...

$ftp->quot("user destination_username destination_password")
[download]

...do this...

$ftp->login("destination_username", "destination_password")
[download]

This will then allow you to ftp through a type 3 firewall (see perldoc Net::Config for definition of a type 3 firewall).

I suppose this was a bit obvious though...

Andrew