can it cause any kind of long term (internal) damage or anything???
I did it recently. I pushed a needle all the way through one nut and came out the other side. Its easier to do that it looks; especially if you use a hypodermic needle as opposed to a sewing needle. Hypodermic needles are designed to pierce flesh. Thus, they easily penetrate the testicle. Larger needles are harder to push through. The pain is a lot less than you would expect. I'm looking for a surgical needle long enough to go through both nuts at the same time.
As far as preparation goes, I showed and shaved before and then swabbed the sack with alcohol. I also used gloves for added measure. There was no risidual pain after the needle was removed.