Jonah Goldberg nails it:

His performance last night confirmed — with an exclamation point — the negative narrative of his entire campaign. Everyone could forgive Ron Paul if he spaced out on the name of a cabinet agency he wanted to shutter, because everyone knows that Ron Paul knows what he knows and has no problem explaining himself under normal circumstances. People are much more unsure about Perry and he compounded that uncertainty last night. It’s fine to say everyone has these bad moments. That’s true. Everyone makes mistakes. What you look for are patterns. Last night was so deadly because Perry reinforced his pattern rather than deviated from it. And he was already on borrowed time.