Lisp runtime implicitly runs each top-level expression as statement. When you run (def x y) it's not only true that this expression returns 'x, it's also true that it alters the static state of the program by adding 'x to the scope.
To make a purely statementless language, I think you need to make everything happen in a single monadic computation. E.g. in pseudo-Haskell-ish:
main : Scope -> StaticIO Scope
main scope0 = do
scope1, x' <- def scope0 "x" Int
scope2, xv <- dec scope1 "x" 3
But the main part is still statement.
