From the comments:
This an existential psychological play was named "The Man in the Cube" and was written and produced by Jim Hensen (prior to his Muppets productions). It was broadcast on NBC's "Experiments in Television" series in 1969. It stars actor Richard Schaal, father of actress Wendy Schaal and one-time husband of Valerie Harper.
My commentary: This is evil and cruelty distilled into a nice, tight, one-hour of suffering. I always knew Jim Henson had a dark, bile-ladened streak of evil tucked inside a green sock-puppet.