Preheat oven to 250 degrees F (120 degrees C). Grease a 10x15-inch baking sheet.
Combine corn cereal, rice cereal, pretzels, and peanuts in a large bowl.
Heat peanut butter, butter, Worcestershire sauce, salt, and garlic powder in a saucepan over high heat until peanut butter is melted, about 5 minutes. Pour mixture over cereal mixture and toss to evenly coat. Spread cereal mixture over prepared baking sheet.
Bake in the preheated oven, stirring every 15 minutes, until cereal mixture is dry, about 1 hour. Allow to cool. Stir candy corn and chocolate pieces into cereal mixture before serving.