Perfidy is classed as a crime because it breaks down the distinction between civilians and combatants in conflicts.
It's essentially a situation where combatants disguise themselves as, say, civilians in some manner in order to gain an unfair advantage in conflicts.
But simply disguising oneself as a civilian isn't in and of itself enough to qualify as perfidy. Undercover operations under both International Humanitarian Law (military) and International Human Rights Law (police) aren't explicitly prohibited.
So perfidy involves significantly more than the mere act of disguising as a civilian, to give an example.
Like I said before, I really wouldn't speculate on precisely what happened in Ibn Sina Hospital. There's too many variables to consider. And a lack of detail.
But you can clearly see the automatic really quick to condemn posts in this thread ahead of information. I've seen this kind of reaction before in other special forces operations and those who are really quick very rarely come to accept what actually happened later on which can be years down the line. In the example I've got in mind, nearly 10 years down the line.
Credible information won't be forthcoming any time soon, I wager.