Reaching with a single segment

the target will be the mouse. You need the distance between x and y axes. Use Math.atan2 for the angle between in radians. Convert to degrees and voila!

package {
    import flash.display.Sprite;
    import flash.events.Event;

    public class UnoSegment extends Sprite
    {
        private var segmentU:Segment;
       
        public function UnoSegment ()
        {
            init();
        }

        private function init():void
        {
            segmentU = new Segment(200, 30)
            addChild(segmentU);
            segmentU.x = stage.stageWidth / 2;
            segmentU.y = stage.stageHeight / 2;

            addEventListener(Event.ENTER_FRAME, onEnterFrame);
        }

        private function onEnterFrame(event:Event):void
        {
            var dx:Number = mouseX - segmentU.x;
            var dy:Number = mouseY - segmentU.y;
            var angle:Number = Math.atan2(dy, dx);
            segmentU.rotation = angle * 180 / Math.PI;
        }
    }
}