package{
    import flash.display.Sprite;

    public class Ball extends Sprite{
        public var speed:int;

        public function Ball(){
            this.graphics.beginFill(0xABCDEF);
            this.graphics.drawCircle(0, 0, 5);
            this.graphics.endFill();

            speed = int(Math.random()*20+1);
        }

        public function destroy(){
            this.graphics.clear();
        }
    }
}
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Frame 1
------------------------

import flash.events.Event;
import flash.display.StageScaleMode;
import flash.display.StageAlign;
import flash.text.TextField;
import flash.text.TextFormat;

stage.scaleMode = StageScaleMode.NO_SCALE;
stage.align = StageAlign.TOP_LEFT;

var txt:TextField = new TextField();
    txt.setTextFormat(new TextFormat("Arial", 12, 0x000000));
    txt.x = txt.y = 5;
    this.addChild(txt);

var balls:Vector.<Ball> = new Vector.<Ball>();
var ball:Ball;
var i:int;

stage.addEventListener(Event.ENTER_FRAME, iterator);
function iterator(event:Event){
    // Create a new ball every frame
    ball = new Ball();
    balls.push(ball);

    // Place ball at random location
    ball.x = stage.stageWidth*Math.random();
    this.addChild(ball);

    // Move existing balls
    for(i=0; i<balls.length; i++){
        balls[i].y += balls[i].speed;

        // Delete ball if out of bounds
        if(balls[i].y > stage.stageHeight){
            this.removeChild(balls[i]);
            balls[i].destroy();
            balls.splice(i, 1);
        }
    }
    txt.text = String(balls.length);
}