I always thought it was a flat out 85 foot drop and at a pretty steep angle, but many people dispute over whether it is 84 feet or 86 feet. For short I just say 85 feet. For other fun info it is about 6 minutes long from over the first hill.
There's also a shoot-the-chutes at Oakwood Park in Wales that's just as steep as Perilous Plunge. It's called Hydro, and unfortunately it too threw a rider to her death ...