From the Super Mario Wiki
The Fake Beanstar is an item that is found in Mario & Luigi: Superstar Saga and Mario & Luigi: Superstar Saga + Bowser's Minions. It is a cheap, plastic version of the Beanstar, given to Mario and Luigi by Prince Peasley, intended to fool Bowletta and Fawful into thinking that it is the real Beanstar so that they cannot use it to steal Princess Peach's voice. However, when the brothers encounter Fawful at Joke's End, he quickly realizes that it is a fake and blasts it with his Headgear, destroying it. He then proceeds to take the real Beanstar from the brothers' suitcase and heads back to Bowletta.