First, I've made an example and then I'll explain it.
For this trick you need two layers, the "inside" layer and the "outside" layer, where your inside and outside objects go respectively. Your magnifying glass sprite will also go on the "outside" layer. On the "outside" layer you'll have a mask object -- placed above the outside box image, but below the magnifying glass image -- that has its blend mode set to "Destination out", which in a basic sense blends out everything below it on its own layer. For the blend mode to work correctly, the "outside" layer needs "Force own texture" set to true in the layer properties.
Let me know if you you have any questions.
EDIT: Looks like I spent too long writing this post, but here it is anyway.