How should I unit test code that uses globally-defined refs?

defref.clj

(def foo (ref 5))
(def bar (ref 7))
(def stuff (ref nil))

(defn update-stuff! [x]
  (dosync
    (ref-set stuff (* x @foo @bar))))

; but this is hard to unit-test

using-binding.clj

; no modification of source code

;-- test code --;

(deftest stuff
  (binding [foo (ref 5)
            bar (ref 6)
            stuff (ref nil)]
    (update-stuff! 8)
    (is (= @stuff 240))))

using-combinators.clj

(def foo (ref 5))
(def bar (ref 7))
(def stuff (ref nil))

(defn update-stuff!-combinator [r_foo r_bar r_stuff x]
  (dosync
    (ref-set r_stuff (* x @r_foo @r_bar))))

(defn update-stuff! [x]
  (update-stuff!-combinator foo bar stuff x))

;-- test code --;

(deftest stuff
  (let [my-stuff (ref nil)]
    (update-stuff!-combinator (ref 5) (ref 6) my-stuff 8)
    (is (= @my-stuff 240))))