Your being extremely unreasonable by suggesting they are at all comparable. One was a simply outright misspeak which did not truly suggest a lack of knowledge at all when looked at in context.
Perry's by contrast clearly is quite serious because it makes it clear he really has little knowledge about the agencies he is advocating outright eliminating, and suggests someone else came up with the list without significant involvement in the decision at all on his part. (Which is a very major policy proposal if he actually goes through with it, especially since the Department of Energy is only one of 15 governmental Department with a Cabinet level head.)
I would view the situation very differently if Perry could have at least said something along the lines of "its something about energy," after the couple of tries at the name. This would have shown he at least knew what the agency he was advocating cutting was involved with. The fact he was unable to do that during the debate for the length of time demonstrated clearly shows the issue here was not merely being able to remember the official name of the agency. (Energy related issues are a basic function of the agency, and he didn't give any impression about being able to remember any of the agency's related major functions either.)