If += takes two arguments (as does +) it should be possible to overload it now, with Perl 5, if you bless one of the two arguments. That is, don't add two lists, but add two list references, at least one of which is a trivial class (you don't need to define any members or anything, just bless it into a package so overloading kicks in).

Now, if you are going to bless things anyway, why not develop it a little into a "numeric vector" class, that supports operator + overloading etc. and members like inner product, absolute value, and other vector things?

Also today you can use mapcar to sum your vectors.

Now back to the incrementing of hash slices: In order to make it worthwhile, you need both a generalized hash slice and a way to apply the ++ to all the members in it. I think functional programming constructs can do a lot of that in Perl 5, and I hope it is fairly simple and natural in Perl 6.

— John