Calling a function only once whenever conditions are correct

I have code which builds up charge while a button is held down and then I want it to execute a function only once. This is within a function called ScanInput() which is called from Update()

while(Input.GetButton("Fire1"))// && (Time.deltaTime > nextFire)) { //Add to the charge as long as the // user is holding the button chargeLevel += Time.deltaTime * chargeSpeed; firedMissile = false; firePlayer001Missile(firedMissile); yield; }

 chargeLevel = 0.0;

Heres the code for the function firePlayer001Missile

function firePlayer001Missile(firedMissile)  
{
if(!firedMissile)
{
    var player001Missile = new Instantiate(currentMissileType, GameObject.Find("Player001ShotSpawnPoint001").transform.position,Quaternion.identity);
currentMissileType.transform.Translate(0,shootForce*Time.deltaTime,0);
player001Missile.rigidbody.AddForce(transform.up*shootForce);
bulletCount++;
firedMissile=true;
}
return firedMissile;
}

As it is now, it fires a barrage of bullets which I think is equal to the number of calls to the function firePlayer001Missile; I want it to fire only one bullet after the fire button has been held down.