In traditional usage, the term does not include crimes that have not yet been solved. Many crimes go unsolved due to insufficient scrutiny, or to investigator incompetence in overlooking vital evidence. In a perfect crime, the perpetrator is often not even a suspect.
A handy and time-tested tool for perfect crime perpetrators (PCP's) is to frame another person or patsy, thus shifting the suspicion away from the PCP. For the naïve or uninitiated this can be quite persuasive.
Students of the JFK assassination know that it was a perfect crime. The guilty assassin met his quick demise, with no trial needed. A video of the shooting shows an already-shot, slumped over Kennedy being shot again from the front and confirms there was at minimum a second shooter involved. The quick killing of Lee Harvey Oswald by Jack Ruby suggests that someone wanted to, and did, shut Oswald up for good. In short, the events point to a frame up, which permitted the perfect crime.
The same-day accusation of Osama Bin Laden for the 9/11 attacks suggests another frame up. All criminal investigations start with the Latin adage "Cui bono?" translated "to whose benefit" suggesting that probable perpetrators often gain something from committing a crime. This makes perfect sense.
When any building is destroyed by fire, a principle suspect is typically the building's owner. The fact that Larry Silverstein carried large insurance policies on the three skyscrapers, the Twin Towers and Building 7, destroyed on 9/11 and that Lucky Larry cashed in on them to the tune of several billion smackers, should make him a prime suspect for any inquiry into that day's events.
Ask who benefits from a crime, and you will likely find the perpetrators. The amount of money made as a result of 9/11 is astounding, so we should all follow the money. None of it was made by Bin Laden. If you want to know who really did 9/11, ask who benefited, cui bono, and you start to get a better idea.