{
  # Make sure file gets closed and unlocked when we're done.
  local *IN;

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

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

  ...
}

##</code><code>##

{
  # Make sure file gets closed and unlocked when we're done.
  local *OUT;

  open(OUT, '>>', ...)
    or die("...: $!\n");

  # Wait for people to stop reading and writting:
  flock(OUT, LOCK_EX)
    or die("...: $!\n");

  # We need to seek if we wish to append,
  # in case people have written to the file
  # before we obtained the lock.
  seek(OUT, SEEK_END, 0)
    or die("...: $!\n");

  ...
}