One use per level question

OK, in my game I want the player to be able to use a powerup once per level. I'm guessing you do this with a Boolean, starting off with it being true, and being false afterwards. I can't get this to work, so here's my script. Where should I put the Boolean, and what should I write? Am I completely wrong altogether?

var prefab : GameObject; function Update() { if(Input.GetButtonUp ("Fire1")) Instantiate( prefab, transform.position, Quaternion.identity ); }

Thanks.

If we assume that the instantiation is the powerup, you can use the boolean as follows:

var prefab : GameObject;
var hasPowerup : boolean = true;

function Update() {

   if(Input.GetButtonUp ("Fire1") && hasPowerup){
      Instantiate( prefab, transform.position, Quaternion.identity );
      hasPowerup = false;
   }

}