I'm using

->get_table(
 Callback => [\&Process_Data,$id],
 baseoid  => '1.3.6.1.2.1.31.1.1.1.x');
[download]

...where x is 1 for the ifname, 6 for the in-bytes-counter64 and 10 for the out-bytes-counter64. You _may_ be using the wrong OID.
You may also want to add "version => 2" to your ->session - call.

You should split the data fetching and processing into two scripts. You'll need "use bigint;" for Perl-calculations with 64bit numbers, but Net::SNMP fails here if I use bigint in the same script. The whole SNMP reply is handled as a string by my script and written using "CONVERT(UNSIGNED BIGINT,'12345')" (for Sybase SQL).