use Carp;
use Fcntl qw(:flock);

sub open_sentinel {
  my $lock_file = shift;
  my $fh = do {local *FOO}; # Old trick
  open ($fh, "> $lock_file") or die "Cannot write $lock_file: $!";
  my $is_locked;
  foreach (1..10) {
    $is_locked = flock($fh, LOCK_EX | LOCK_NB);
    last if $is_locked;
    select (undef, undef, undef, 0.01);
  }
  return $is_locked ? $fh : undef;
}