m.target
m.currentTarget 

import flash.display.MovieClip;
import flash.events.MouseEvent;

var holder:MovieClip = new MovieClip();
holder.name = 'holder';
holder.graphics.beginFill(0xff00aa,.2);
holder.graphics.drawRect(0,0,400,300);
holder.graphics.endFill();
holder.addEventListener(MouseEvent.MOUSE_UP, mouseUpHandler);

var cube:MovieClip = new MovieClip();
cube.name = 'cube';
cube.graphics.beginFill(0x333333);
cube.graphics.drawRect(0,0,50,50);
cube.graphics.endFill();
cube.x = cube.y = 100;

var ring:MovieClip = new MovieClip();
ring.name = 'ring';
ring.graphics.beginFill(0x000000);
ring.graphics.drawCircle(0,0,30);
ring.graphics.endFill();
ring.x = 150;
cube.addChild(ring);
holder.addChild(cube);
addChild(holder);

function mouseUpHandler(m:MouseEvent):void{
    trace(m.currentTarget.name + ' - ' + m.target.name);
}