This definitely looks and feels like a genuine standing stone. It's not marked on the OS map from the 1860s (see link below), but a lot of sites in Perthshire weren't. I believe that this is a real standing stone, and if it isn't, it's a brilliant fake.
Balnakeilly Standing Stone
So- is it a megalithic monument or not? According to the RCAHMS database this stone may have been erected in the mid 19th C to adorn the entrance to Balnakeilly House. Apparently the general regularity of the stone makes it appear 'modern'- I'm not entirely sure- I've seen many other monoliths as regularly shaped as this one- it's also set back from the road quite a bit and in a group of trees. If it were to adorn the entrance it would have been given more prominence methinks. The stone itself is over 2 m high. I had a quick look at the new OS Explorer map and noticed that it's not marked in antiquarian font, although all previous editions have marked it as so. Hmmmm.
Directions - head N from Perth on the A9 (sign-posted Inverness). After about 40.0km, take the turn-off to the left for Pitlochry. This road (the A924) takes you back under the A9, and follows the River Tummel into the centre of Pitlochry. About 500m after passing under the railway bridge, take the road up to your right (sign-posted A924 & Moulin) and go straight over the mini-roundabout into Moulin. Park opposite the Moulin Inn, and walk continue walking up the road for 200m to the gates of Balnakeilly House - you can't miss the gates, but it's easy to miss the stone camouflaged amongst the trees! It's on the left of the gates as you look at them.