If you want to position file for sysread() or syswrite(), don't use seek() -- buffering makes its effect on the file's system position unpredictable and non-portable. Use sysseek() instead.

On some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:

    seek(TEST,0,1);

This is also useful for applications emulating tail -f. Once you hit EOF on your read, and then sleep for a while, you might have to stick in a seek() to reset things. The seek() doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <FILE> makes Perl try again to read something. We hope.

If that doesn't work (some stdios are particularly cantankerous), then you may need something more like this:

    for (;;) {
        for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
            # search for some stuff and put it into files
        }
        sleep($for_a_while);
        seek(FILE, $curpos, 0);
    }