I thought of that, but dismissed it. He could have named $( something different if he wanted to use parens for variables, or did "${var}" come later? I figured it had been in Perl from the start due to its parallel in bourne.
I agree that ${...} is certainly originally in Perl because of the familiarity of the syntax in shell. But Perl is willing to change syntax if a new one is clearer - witness the switch from ' to :: as a package separator.
But my understanding is that references and most of our dereferencing syntax was added in Perl 5. Which means that backwards compatibility would prevent the $(...) syntax from being introduced after that.