New invariant sigils in Perl 6
Wouter, apparently most people found Perl's changing sigils (that's the name of Perl's paraphernalia: $, %, @) annoying. That's the reason why they are becoming invariant with Perl 6.
Actually I assume that's the reason; I haven't read the corresponding RFC, partly because it is usually impossible to find the one you are looking for ...
BTW, I recommend to all people interested in Perl to have a look at Patrick Michaud's slides on Perl 6: I quite enjoyed his talk at FOSDEM, way better than Larry Wall's talks I've attended in the past (this time at least I went back home with some actual content, instead of only LOLing for the talk duration).