{
  # Make sure file gets closed and unlocked when we're done.

  open(my $IN, '<', ...)
    or die("...: $!\n");

  # Wait for people to stop writting:
  flock($IN, LOCK_SH)
    or die("...: $!\n");

  while (my $line = <$IN>) {
    ...
  }
#  close ($IN); # not really needed => closed at end of block
}