Cannot get mouse position in 2D using perspective

I am trying to make a side-on (would call it 2D but I guess it's not since I'm using perspective) game using the perspective camera to show depth. In orthographic mode my code works fine, the character will fire a ray between itself and the mouse cursor, but when I switch to perspective camera, this stops working.

I can only assume it's because using perspective it treats it as a vector3, rather than the vector2 I need... I'm stuck on how to achieve the same effect using a perspective camera.

This is my code that works using the orthographic camera:

     void Shoot() 
     {
         //translate mouse position into world coordinates
         Vector2 mouseposition = new Vector2 (Camera.main.ScreenToWorldPoint (Input.mousePosition).x, Camera.main.ScreenToWorldPoint (Input.mousePosition).y);
 
         Vector2 firePointPosition = new Vector2 (firePoint.position.x, firePoint.position.y);
         RaycastHit2D hit = Physics2D.Raycast (firePointPosition, mouseposition-firePointPosition, bulletRange, whatToHit);
         if (Time.time >= timeToSpawnEffect) 
         {
             Effect();
             timeToSpawnEffect = Time.time + 1/effectSpawnRate;
         }
         Effect();
         Debug.DrawLine (firePointPosition, (mouseposition-firePointPosition)*bulletRange);
         if (hit.collider != null) 
         {
             //Debug.DrawLine (firePointPosition, hit.point, Color.red);
             //Debug.Log ("Hit " + hit.collider.name + " for " + damage + " Damage.");
 
             EnemyController enemy = hit.collider.GetComponent<EnemyController>();
 
             if (enemy!= null)
             {
                 enemy.DamageEnemy (damage);
             }
         }
     }