There's a fine "hit counter" example in the File Locking section at the bottom of perlopentut:

           use Fcntl qw(:DEFAULT :flock);

           sysopen(FH, "numfile", O_RDWR | O_CREAT)
               or die "can't open numfile: $!";
           # autoflush FH
           $ofh = select(FH); $| = 1; select ($ofh);
           flock(FH, LOCK_EX)
               or die "can't write-lock numfile: $!";

           $num = <FH> || 0;
           seek(FH, 0, 0)
               or die "can't rewind numfile : $!";
           print FH $num+1, "\n"
               or die "can't write numfile: $!";

           truncate(FH, tell(FH))
               or die "can't truncate numfile: $!";
           close(FH)
               or die "can't close numfile: $!";
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