Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Spray a bundt pan with nonstick spray.
Evenly pour the melted butter into the bottom of the bundt pan and top with the brown sugar.
Cut the pineapple rings in half and, alternating with the cherries, place into the butter-brown sugar mixture in the bottom of the pan.
In a large bowl, combine the cake and pudding mixes together. Add the reserved pineapple juice and milk. Add the eggs and vegetable oil, and mix until fully blended.
Pour the batter evenly over the pineapple and cherries.
Bake for 30 minutes, following the cake mix instructions for a bundt pan, until an inserted toothpick comes out clean.
Cool the cake for about 10 minutes before running a knife around the edges and inner ring of the pan. Invert onto a serving plate and slice between pineapple rings to serve.
When you add the reserved pineapple juice and milk in step 4, they should equal 1 cup of liquid. If you like, you can first pour the reserved pineapple juice into a liquid measuring cup. Add enough milk until you have 1 cup of liquid. Add the liquid to the combined cake-and-pudding mix, then proceed with the recipe.