How do I do a tail -f in perl?

    seek(GWFILE, 0, 1);

The statement seek(GWFILE, 0, 1) doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <GWFILE> makes Perl try again to read something.

If that doesn't work (it relies on features of your stdio implementation), then you need something more like this:

        for (;;) {
          for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
            # search for some stuff and put it into files
          }
          # sleep for a while
          seek(GWFILE, $curpos, 0);  # seek to where we had been
        }

If this still doesn't work, look into the POSIX module. POSIX defines the clearerr() method, which can remove the end of file condition on a filehandle. The method: read until end of file, clearerr(), read some more. Lather, rinse, repeat.