There are several things wrong there that are fixed in Perl 6. You won't need the plus because you'll disambiguate autoquotes differently, so that a shift in curlies is always a real shift. And plus is always numeric in Perl 6--it's not a no-op as it is in Perl 5. But mostly, methods just look different. Your code might look more like this in Perl 6:

method foo ($self: $key) {
    .{$key} = 3;
    return $self;
}
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As for vec, that interface is a total kludge. You'll be much happier with real bit arrays, I expect.