McDarren is right, the snmp port (by default, and if and only if you didn't change where the server is listening) is the 161.

Anyway, I think you don't need to install IO::Socket::INET6 support to get your script running. You only need to know if your snmp server (or better, your target agent) is listening at port 161 over udp or tcp.
You can change the default behaviour (by default server and clients use udp) of your perl script snmp client, by specifying the 'domain' option (see Net::SNMP)

good luck!