From the Super Mario Wiki
The Return Pipe is an item in Super Paper Mario.
It is given to Mario by Merlon at the start of the game after Mario places the red Pure Heart on the Heart Pillar. It resembles a red Warp Pipe with white stripes on it. The function of the Return Pipe is to return the player back to Flipside from any dimension at most times. However, in some places, such as Sammer's Kingdom or in The Underwhere, the Return Pipe cannot be used. The Return Pipe also can lead to Flopside if used at Castle Bleck.
Names in other languages
||Pipa del Retorno
||Tuyau de Retour