According to Montresor, what makes a perfect crime in "The Cask of Amontillado"?

Montresor feels like a crime is not the perfect crime unless you get away with it. 
This story is about a man who meticulously plans and carries out another man’s murder. He gives no reason for the murder except for some unmentioned, vague “injuries” of which we get no specifics. Nonetheless, Montresor feels he has cause to murder Fortunato.  

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. 

Basically, what Montresor is saying here is that if he does not get away with the murder, he will have failed to achieve perfect revenge. Therefore, he plans to get Fortunato at Carnival time, when he will be drinking and feeling lighthearted. This will allow Montresor to catch him off guard. To lure Fortunato in, Montresor tells him he has a special cask of wine for him to evaluate.
Montresor’s plan is to take Fortunato into the catacombs and brick him into the wall. By the time Fortunato realizes what is happening, it is too late. At the end of the story, we learn it has been fifty years since Montresor bricked Fortunato up into the wall, and he has apparently been able to get away with it.

I hastened to make an end of my labour. I forced the last stone into its position; I plastered it up. Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them.

It seems Montresor did get away with it. So who is he telling the story to? Does he have a guilty conscience? What is the significance of the line “In pace requiescat” (rest in peace)? Is it intended ironically, or does Montresor feel bad? We can only speculate answers to these questions.