Note how both XXX() and +XXX can be used as the hash key but only XXX() works on the LHS of the fat comma

Note that neither XXX() nor +XXX is quoted as a hash key - what is between the parens isn't a bare word. In all cases, the key is "a".

There's nothing to autoquote in XXX() => 123 as there's no bare word directly left of the arrow. However, in +XXX => 123 there is a bare word left of the arrow: hence it's tokenized as +"XXX", 123.