See the documentation on seek, which has an example of how to "tail" a file:

        for (;;) {
            for ($curpos = tell($fh); $_ = readline($fh);
                 $curpos = tell($fh)) {
                # search for some stuff and put it into files
            }
            sleep($for_a_while);
            seek($fh, $curpos, 0);
        }
[download]

You will either set $/ to your record size so that readline does its magic or simply read your records yourself instead of using readline.