I'm not sure what you mean. But I know that $1 and friends are block scoped, meaning that it's safe, and has always been safe, to nest substitutions provided the inner substitution was in a block.

Calling a sub is one safe way to achieve that. It also has the added advantage that you don't have to take care of having to escape any unpaired nested braces: the substitution part may look like a code block to us, but it actually isn't.