How do I do a tail -f in perl?

First try 

    seek(GWFILE, 0, 1);

The statement seek 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.

head2 How do I dup a filehandle in Perl?

If you check open, you'll see that several of the ways to call open should do the trick. For example: 

    open(LOG, ">>/tmp/logfile");
    open(STDERR, ">&LOG");

Or even with a literal numeric descriptor: 

   $fd = $ENV{MHCONTEXTFD};
   open(MHCONTEXT, "<&=$fd");	# like fdopen(3S)

Error checking has been left as an exercise for the reader.

head2 How do I close a file descriptor by number?

This should rarely be necessary, as the Perl close function is to be used for things that Perl opened itself, even if it was a dup of a numeric descriptor, as with MHCONTEXT above. But if you really have to, you may be able to do this: 

    require 'sys/syscall.ph';
    $rc = syscall(&SYS_close, $fd + 0);  # must force numeric
    die "can't sysclose $fd: $!" unless $rc == -1;