...
  my $active = 1;
  my $timeout = 20;
  my $an;

  open my $fh, "tcpdump -nn -l -e dst 224.111.111.112 and not igmp |";
  $read_set->add($fh);

  while ($active) {
    my ($rh_set) = IO::Select->select($read_set, undef, undef, 1);

    if (defined $rh_set) {
      if ($_ = <$fh>) {
        # do some parsing on the line
        $an = $1;
        last;
      } else {
        # tcpdump died. real code will sleep 2 seconds
        # and then close and reopen $fh here
      }
    }
    if (--$timeout == 0) { $active=0; }
  }
  close $fh;