How do you know what $_ really is in your program?

I can see that get printed on milk cartons one day :) (Talk to your children! A message from the government of Canada.)

Anyway, all kidding aside, here's what I'm doing, and now It Just Works, so thank you all for your thoughts and input. The calling sub:

#... blah blah blah
            open(FH, $opts{-file});
            $opts{-handle} = *FH;
            return $parser->from_handle(%opts);
#... la dee da
[download]

And the callee:

#... stick out left leg
    while ( readline($opts{-handle}) ) {
        if ( $_ ) {
            # ... frobnicate
        }
    }
# return right leg
[download]