Some things to check:
in reply to net::dns::update and in-addr.arpa ??
- Do you have permission to update that zone (the w.z.y.z.in-addr.arpa zone)
- What are $host, $ip and $type? I guess the important one here is $type. Is it "PTR"?
- What is the value of $DnsDomain? Is it something like ".foo.com." (with the ".")
- Finally, should the Update->new() call refer to $DnsDomain or $revZone? I am not sure as I have not used Net::DNS::Update.