Booldals is correct. No Torque function can modify the variable pushed to it unless it is global. Even then, it creates a local copy of it.
%a = "1 1 1";
%a = setWord(%a, 1, "3");
echo(%a);
Similarly, doing this:
function addOne(%num) { %num = %num + 1; }
%x = 1;
addOne(%x);
echo(%x);
will echo 1.