Curses is arguably the simplest GUI and there are a number of modules available to interface with it.

There is Curses which offers a simple procedural interface and is a very long-standing module. You'll find lots of sample scripts for this on the web, but some will be very old.

For OO there's Curses::UI, but that hasn't seen an update in 5 years and has quite a few outstanding issues.

Plus others I haven't looked at: Curses::Application, Curses::Simp, etc.