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

loop
iter.  step         etime=       petime=
...

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

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

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