One of my favorite fun sayings- but with some thought it falls apart. "Perfect" is somewhat vague, and the argument could be made the "perfect" crime from the criminals perspective would be one you don't have to take the effort to hide. A good example would be certain high profile cases of abuse of power or status, where the perpetrator was able to enjoy the benefit she of the crime and faced no consequence. Like some crimes where the perpetrator wasn't found until old and feeble, and deemed not worth a trial. Certain crimes under diplomatic immunity would work too. If they aren't severe enough you won't be deported and unless a stink is made won't even face reprimand or consequence often times. A skillful crime is one no one discovers, but the powerful can do what they want in plain sight.
Can you keep it?
Swear this one you'll save
In your pocket
Taking this one to the grave
Then I know you
Won't tell what I've said
do you smile
like you've been told a secret?
Spelled wrong.