%~ is not available for your use. Punctuation variables are reserved for perl's use. The ^_ namespace is reserved for this use. The closest available analogue of %~ is %{'^_~'} because %^_~ is a syntax error. I'd suggest %^_C to follow the \C{name} theme.

Hashes are cleared by assigning an empty list, not by undefining them. When you say %hash = () you allow perl to be smart about the allocation of the memory associated with %hash. undef %hash circumvents this and forces some unnecessary work.

I deliberately placed the new syntax to the right of the capture because otherwise I would have had to do some balanced delimiter matching. perlop covers the requirements for matching (...) in regexps. It is possible, I just couldn't do it in the two minutes it took to write the initial example.

The implication of allowing $~{EXPR} to inform the creation of the hash key is that you must allow arbitrary perl code inside EXPR. This is not a problem if you take into account the same balanced-tag handling already mentioned in perlop.

To do this really well requires Text::Balanced and an understanding of Gory details of parsing quoted constructs from perlop.