I still don't get locking.  I just want to increment the
       number in the file.  How can I do this?

       Didn't anyone ever tell you web-page hit counters were
       useless?  They don't count number of hits, they're a waste
       of time, and they serve only to stroke the writer's van-
       ity.  It's better to pick a random number; they're more
       realistic.

       Anyway, this is what you can do if you can't help your-
       self.

      use Fcntl qw(:DEFAULT :flock);
      sysopen(FH, "numfile", O_RDWR|O_CREAT)       or die "can't opennumfile: $!";
      flock(FH, LOCK_EX)                           or die "can't flock numfile: $!";
      $num = <FH> || 0;
      seek(FH, 0, 0)                               or die "can't rewind numfile: $!";
      truncate(FH, 0)                              or die "can't truncate numfile: $!";
      (print FH $num+1, "\n")                      or die "can't write numfile: $!";
      close FH                                     or die "can't close numfile: $!";

       Here's a much better web-page hit counter:

           $hits = int( (time() - 850_000_000) / rand(1_000) );