From the Super Mario Wiki
The Plumber's Periscope is, as its name implies, a periscope used in plumbing; it is also owned by Luigi, who seemingly brought it with him to the Mushroom Kingdom. It appears in the The Adventures of Super Mario Bros. 3
In The Adventures of Super Mario Bros. 3 episode "Recycled Koopa", Toad, suspicious of Bowser and the Koopalings, brought out Luigi's Plumber's Periscope in a cardboard box, which he gave to Mario to spy on the Koopalings and Bowser. With Luigi's Plumber's Periscope, Mario managed to look into a nearby Warp Pipe and see that Bully, Kootie Pie and Big Mouth Koopa were employing Sledge Brothers to dump garbage through a Warp Pipe. Seeing the Koopalings doing this, Mario and Luigi jumped through the Warp Pipe to Bowser's Castle, planning to stop the Koopalings and their polluting.