Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 10-inch fluted tube pan (such as Bundt(R)).
Beat yellow cake mix, eggs, instant pudding mix, water, pineapple juice, oil and poppy seeds together in a large bowl; pour into prepared pan.
Bake in preheated oven until a toothpick inserted into the center of the cake comes out clean, about 45 minutes.
Remove cake from oven, rest on wire cooling rack. Run knife between cake and sides of pan to release. Using a skewer, poke holes in the cake at 1/2-inch intervals. Pour sauce over cake and let rest in pan for 45 minutes to cool.
Stir sugar, pineapple juice, and butter together in a saucepan over medium heat until the butter melts, 1 to 2 minutes; bring to a boil and cook for 1 minute more. Remove saucepan from heat and pour sauce over the cake.
Let cake cool and absorb the sauce at room temperature for at least 45 minutes.
Remove cake from pan and dust with confectioners' sugar.