How to have score when an object is destroyed

If you will have only one score variable for the whole game, declare it as static like this:

 static var score: int;
 
 function OnMouseDown () {
   Destroy(gameObject);}
   score++; // increment score
 }

Supposing that this script is called BubbleScript.js, for instance, you can access the static variable score in any other script as BubbleScript.score. If you have a GUIText that shows the current score, its script could be like this:

 private var lastScore: int; // used to detect when the score changes
 
 function Update(){
   // only change the GUIText string when the score has changed
   // this avoids unnecessary memory allocation, improving performance
   if (BubbleScript.score != lastScore){ // if score changed...
     guiText.text = "Score: " + BubbleScript.score; // update GUIText...
     lastScore = BubbleScript.score; // and lastScore
   }
 }

NOTE: Be aware that static variables are created only once when the game starts and survive to level changes, thus score won't be reset when changing to other levels - you must explicitly assign zero to it when you want to clear it.