The main problem is that the opcodes into which Perl is compiled expect scalars. You would literally need to rewrite the entire interpreter, which is massive! (Remember, each "function" in perlfunc is an operator, and they basically correspond one-to-one to an opcode.)

Similarly, subs expect scalars. Would these primitives get upgrade to scalars when passed to a sub? If so, that's going to seriously limit the amount of benefit you can get. If not, you need to massively change how sub calls are made too.

Also, there's a huge number of unresolved problems with your proposal. The most predominant is that you mention getting rid of reference counting for these variables, but you don't mention what garbage collection scheme would you use instead or how it would help.

These changes are so massive, it makes more sense to talk about spinning off a new language.