From the Super Mario Wiki
Harhall's Studio is a studio in Mario & Luigi: Superstar Saga where Harhall works on clothing designs. It is located on the southern border of the Beanbean Kingdom. The studio has a water pump and three pipes from which Designbombs emerge. On the small island, there is a shirt which is used for Splart designing. There is also a cage hanging from a tree where Harhall keeps a piece of the Beanstar. It is from here that the minigame Splart can be played.
Mario and Luigi come here to locate a piece of the Beanstar. Upon arriving, though, Harhall tells them that he will only give the Beanstar piece to employees. Mario and Luigi take Harhall's job offer, and manage to create all of the designs that Harhall wants, thus earning the piece of the Beanstar.
Mario and Luigi can come back at a later point and continue their designing. With each new design, Harhall awards the two with a single Woo Bean. After designing all possible clothing combinations, Mario and Luigi receive the Harhall Jeans for their efforts, at which point Harhall leaves the studio for the remainder of the game.