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.