my $cmdoutput = `snmpwalk blah blah blah | tail -1`;
 my $snmpval = substr( $cmdoutput, 54, 10 );

##</code><code>##

loop
iter.  step         etime=       petime=
----------------------------------------
   1   at "#time"   time1        0
       at "#eth0in" time1        time1

   2   at "#time"   time2        time1
       at "#eth0in" time2-time1  time2

   3   at "#time"   time3        time2
       at "#eth0in" time3-time2  time3

##</code><code>##

%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...