The question is: What are you expecting it to print? If you are expecting a 'used only once' warning, these don't show up for all the special variables composed of only a special char, like %! or %^ or @# ...
Aha, which it doesn't do here. As was figured out in the meantime here in this thread this behaviour changed between perl 5.6 and 5.8. And as you can guess by now, I'm on 5.6 :) - therefore my question. I get the same results as bbfu below.