Heat the oven to 350°F and butter a 9-by-13-inch baking dish.
Spread the pineapple with its juice across the bottom, then distribute the peaches over it and sprinkle with brown sugar.
Cover the fruit with an even layer of dry cake mix without stirring.
Scatter the pecans across the mix and drizzle melted butter slowly over the whole surface.
Bake for 45 to 50 minutes until the top is deep golden with no powdery patches and fruit bubbles at the edges.
Rest for 25 minutes before serving so the pineapple-peach filling settles.