The decision in Edwards vs. Aguillard helped inspire the Intelligent Design movement. In the 7-2 decision, the majority opinion contained a caveats: "We do not imply that a legislature could never require that scientific critiques of prevailing scientific theories be taught [and] teaching a variety of scientific theories about the origins of humankind to schoolchildren might be validly done with the clear secular intent of enhancing the effectiveness of science instruction."
It's plausible that Perry misspoke, and meant to say Texas schools teach both evolution and Intelligent Design -- but even that doesn't quite match the facts.
Conservatives on the Texas Board of Education wanted to greenlight supplemental teaching materials that give weight to Intelligent Design. However, the board recently approved materials silent on Intelligent Design, but that critics charge could pave the way for invalid critiques of evolution nonetheless.