Only constructs that are actual matchers look for following sigspace. Colon declarations, opening brackets, disjunctions/conjunctions, and closures specifically do not. The beginning of a rule does not, so the TOP rule may need to account for leading whitespace. (A ^ followed by whitespace is sufficient for this.)
ingy++ requested a method of stopping optionally after the separator, so that's now %% (with some notion of "evenly divisible" in some kind of loose sense). (Note, if you want to require the separator at the end, don't use this construct, just use [x ',']* or such. sorear++ requested clarification: % (and %%) are not really quantifier modifiers; they modify a quantified atom to insert requirements between each iteration of that quantified atom. Hence any quantifier may still take an additional control quantmod in addition to a separator.
The separator syntax is now required to be a modifier on a quantifier, so the old <a> ** ',' is now written <a>+ % ','. This allows for other quantifiers like * or ** on the left as the basis of the constraint, while disambiguating use of ** to only work as a quantifier without overloading separator matching. The C<%> because it can be pronounced 'modulo' and because it looks like the relationship of two things. And also because it's unlikely to be confused with other regex forms.
I have been told on #perl6 that the old names are unclear, this seems to be the new consensus