According to Montresor, what makes a perfect crime?

In the opening paragraph of Edgar Allan Poe's short story "The Cask of Amontillado," the first-person narrator Montresor vows revenge against the perceived insults he has endured at the hands of Fortunato. Montresor has planned to murder his so-called friend. For Montresor, the crime of killing Fortunato can only be considered a perfect crime if two things are part of the plan. First, the crime must be undetected; second, the victim must understand exactly who is perpetrating the crime against him. Montresor's crime should go unnoticed and, moreover, he needs Fortunato to recognize Montresor is the one killing him. He indicates this by saying,

I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong. 

In the first sentence of this statement, Montresor uses the word "impunity," which is defined as being without fear of punishment. In the third sentence, he makes it clear that, as the "avenger," he needs to make certain the victim understands exactly what is happening. In the end, Montresor's plan works to perfection, and he claims in the final line that the bones of Fortunato were never discovered.