The rule is not really immutability, but referential transparency. It's perfectly OK to use locally declared mutable variables and arrays, because none of the effects are observable to any other parts of the overall program.
The principle of referential transparency (RT) is this:
An expression e is referentially transparent if for all programs p every occurrence of e in p can be replaced with the result of evaluating e, without affecting the observable result of p.
Note that if e creates and mutates some local state, it doesn't violate RT since nobody can observe this happening.
That said, I very much doubt that your implementation is any more straightforward with vars.