A good substitute for a paper piercer is a toothpick!

If you want to use a brad(s) on your project you should always "pre-drill" (LOL) the hole before you attach the brad.

I would suggest using your exacto knife to make an extremely small slice where you want the brad to go.

Then, take your paper piercer, or if you don't have a paper piercer, use a toothpick, to poke the hole where you want the brad to go!