...

# Open the SNMP session and query in the background.
$sess = new SNMP::Session(DestHost => $thisip, 
        Community => $$optionsref{community}, 
        Retries => $$optionsref{retries}, 
        Timeout => $$optionsref{timeout}, 
        Version => '1');
$mib = 'sysDescr';
$vb = new SNMP::Varbind([$mib]);

# The responses to our queries are stored in %list.
$var = $sess->getnext($vb, [ \&gotit, $thisip, \%list ]);

# Update the rate limiting counter.
$count++;

# After every 100 IP's, wait for the timeout period (default is two seconds) to keep from overwhelming routers with ARP queries.
if ( $count > '100' ) {
  &SNMP::MainLoop($looptimer);
  $count = 0;
}

# Increment the IP address for the next pass.
$intip++;

...

# This is the little function called by SNMP::MainLoop when a callback comes in.  It just stuffs the value of the response (if any) into the appropriate place in %list and returns.
sub gotit {
  my $myip = shift;
  my $listref = shift;
  my $vl = shift;
  if ( defined $$vl[0] ) {
    $$listref{$myip}{desc} = $$vl[0]->val;
  }
  return();
}