%iface = ( eth0in => { param => 'InOctets.3',  now => 0, prv => 0 },
           eth0out=> { param => 'OutOctets.3', now => 0, prv => 0 },
           loout  => { param => 'OutOctets.1', now => 0, prv => 0 },
          );

# use $iface{eth0in}{param} to run snmpwalk for that item,
# use $iface{ethoin}{now} to store snmpwalk output, etc...