The lens does not change perspective. The camera position does.

An object of height h at distance d subtends h/d at the lens. Two objects, one near and one far, appear in the ratio (h_n/h_f) × (d_f/d_n). There is no focal length in that expression. Change the lens and both objects scale by the same amount, so their ratio — which is the whole of what perspective means — cannot move. Two frames from one spot are the same picture at two magnifications, and the longer one is a literal crop of the wider.

The engine is free and needs no account. So why does an 85 mm portrait look unlike a 24 mm one? Because of where you stood. Holding a head at 60% of frame height puts the 24 at 40 cm and the 85 at 1.42 m — 3.54× apart, which is exactly 85/24. At 40 cm a face renders 25% larger at the front than the back; at 1.42 m it renders 7%. That is the whole of what gets called lens compression, and this page computes it without reference to any lens: put the 85 at 40 cm and it distorts the face exactly like the 24 does, and simply frames a tighter crop. A framing and a focal length together determine a distance — which is also why a room decides a shot list. Every lane ships a worked example that costs nothing. Writing the report needs credits.