Things are a little desperate when you reply to your own questions:
Okay, I've looked at the source to FTP.pm and note that when deleting, we simply pass the 1st argument (hopefully a filename) to the ftp DELE command, therefore the fault is either with the FTP server (SCO Openserver, WU-FTPD v2.1 - ouch ... patch that baby!) or with the fact that we're having to do this over a passive session over port-forwarding connection.
Anybody have any issues with using ftp over a passive session?