Very nice code, much simpler & shorter than I expected. I tried to add an = (Perl or C internal-to-expression assignment) to the operators, though, lowest precedence & right-associative:

I added it to the operator pattern:

and kind of defined the LHS variable

but it fails because Function::evaluate tries to coerce the value, which I set to the variable name, into a number:

I think I have a workaround, but it looks like '=' is a special case because the LHS variable should not be evaluated before calling the assignment function.