I meant "perldoc perlop", but it doesn't matter, I somewhat misunderstood anyway. since qr// are quote operators, I think this makes perfect sense, given the way the feature is supposed to work. $T represents a compiled regex, but an empty one. When the interpreter sees that it's an empty regex, it attempts to use the last successfully matched one (as it's supposed to do).
Re^2: Empty qr// fails to match -- Is this known bug?