One updates the last modification and access times of a plain file (creating it if it doesn't exist); the other makes a named pipe in the filesystem.

Named pipes are pretty much the same as those returned by the pipe system call, except that the endpoint is accessible from the filesystem. This allows processes without a parent-child relationship to get access to the same pipe pair. I believe perlipc covers using pipe pairs, and any decent *NIX programming tome (Stevens, Rockhind) should cover them in more detail.

You'll find an explanation with man fifo on you host, or on the web here : http://www.die.net/doc/linux/man/man4/fifo.4.html

    chdir; # go home
    $FIFO = '.signature';
    $ENV{PATH} .= ":/etc:/usr/games";


    while (1) {
        unless (-p $FIFO) {
            unlink $FIFO;
            system('mknod', $FIFO, 'p')
                && die "can't mknod $FIFO: $!";
        }


        # next line blocks until there's a reader
        open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
        print FIFO "John Smith (smith\@host.org\n";
        close FIFO;
        sleep 2;    # to avoid dup signals
    }
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