Rather than fight where the mod perl was killing off the password athentication, I just went to this:

   my @back = `/usr/bin/perl sftptest.cgi file=$fullfile\\&name=$thisf
+ilename`;
[download]

Thanks to the author of Net::SFTP::Foreign for all his help in this matter, but it's a config issue with the servers. I hope this helps anyone else facing the same issue.