use SNMP;
my $session = SNMP::Session->new( ... );

sub callback {
  if ( !defined( $_[0] ) {
    warn "request timed out: $session->{ErrorStr}\n";
  }
  # do something with VarbindList $_[0]
}

$session->get( [[ $oid ]], \&callback );

SNMP::MainLoop();