my $pid = open(T, "tcpdump ...|");
eval {
  while (1) {
     $SIG{ALRM} = sub { die "time to stop" }
     alarm(20);
     my $line = <T>; # read a line from tcpdump
     alarm(0);
     last unless defined($line);
     # process line
  }
};
kill 9, $pid;
close(T);