I have found a secret warp pipe in world 1-2 that takes you to World 2. To find it, you need a cat suit. Go to the part where you can find he warp pipe that leads to the stamp but don't go in. Go to the right of it and climb with your cat suit. If you manage to get to the top, you will find a orange warp pipe that you can go in. It will take you to World 2 and you can get back with the map!