List of Vector2 - Madness looms in simple script

First t$$anonymous$$s usually doesn't work:

 var arr = new float[100];
 for (val in arr)
 {
    val = Random.value;
 }

I don't use UnityScript, but a foreach / for-in loop uses an IEnumerator to iterate through the array. You can never assign values to the iterator variable since it's always just a copy of the value stored in the array. Use a normal for loop:

 var arr = new float[100];
 for (var n = 0; n< arr.Length; n++)
 {
    arr[n] = Random.value;
 }

Second, your problem that the returned values are 0,0 is that Vector2 is a value type. When you use the indexer of your List you get a copy of the item. Changing the members of t$$anonymous$$s copy won't change the item in the list.

Use either :

 for(var i:int=0;i<arr.length;i++)
 {
    if(arr[i]>tempMaxList[0].y)
    {
       tempMaxList[0] = new Vector2(i,arr[i]);
    }
 }

or

 for(var i:int=0;i<arr.length;i++)
 {
    if(arr[i]>tempMaxList[0].y)
    {
       var tmp = tempMaxList[0];
       tmp.x = i;
       tmp.y = arr[i];
       tempMaxList[0] = tmp;
    }
 }

Lastly, I don't get why you use a List. You just add one item and always use t$$anonymous$$s like a single variable. So the whole t$$anonymous$$ng could be done like t$$anonymous$$s:

 function Test()
 {
     var arr = new float[100];
     for (var n = 0; n< arr.Length; n++)
     {
        arr[n] = Random.value;
     }
 
     var tempMax = Vector2.zero;
 
     for(var i = 0; i < arr.length; i++)
     {
        if(arr[i] > tempMax.y)
        {
           tempMax.x = i;
           tempMax.y = arr[i]; 
           Debug.Log(tempMax + " " + i + " " + arr[i]);
        }
     }
 }