use Fcntl qw(LOCK_EX LOCK_UN);

#select H; $|=1; select STDOUT;  # Uncomment to fix
$| = 1;

open H,'>','foo.txt' or die "Can't open, $!\n";


print "Opened filehandle. Forking into 2\n";
my $pid=fork;
if (!defined $pid) {die "fork failed\n"}
elsif ($pid) {
  print "Parent will lock, wait a couple seconds, and exit\n";
  flock H,LOCK_EX;
  print "Parent has lock\n";
  print H "Parent writes to file\n"; # Comment this out to "fix"
  sleep 2;
  flock H, LOCK_UN;
  print "Parent byebye\n";
  exit;
}
else {
  sleep 1;
  print "Child hi\n";
  flock H,LOCK_EX;
  print "Child has lock\n";
  flock H, LOCK_UN;
  print "I am the free child. Bye now!\n";
  exit;
}