I'm a U.S. native. I've NEVER heard "darl." Never. It actually is difficult and unnatural to pronounce. It doesn't feel right to stop after an "-rl." Even though it's longer, it is actually EASIER to say "darling" (or "darlin'"). "Darling" is common. It is culturally COMPLICATED. Usage is regional or local or personal. I don't think you should use it. It's not a bad word and it won't insult anyone, but it will sound incongruous and funny to hear a foreign speaker use it. --I COULD call my wife "darling," but the term of endearment I actually use is "sweetie." --My first thought was EXACTLY the same as Su.Ki. It is a word that might be used fifty-year-old waitress in a diner talking to customers. --It is used by parents and grandparents admiring cute children, or a puppy, or a laughing at a baby panda sneezing on YouTube: "Oh, isn't he a darling!" --If we visit friends and bring a house gift, I can imagine the wife saying something like this: "Oh, you brought me a chrysanthemum, well, aren't you a darling!" Then she would probably gives me a quick "friendly' kiss, a "peck" on the cheek.

We do use "darl" (darling) in Australia, but as Su.Ki. has said, we'd expect to hear it from older women, who would also use terms of endearment such as "love" and "dear(ie)". In Australian English, the "-ar-" part is pronounced as a long "aa", so you'd hear something like "daal" down here. That makes it easier to pronounce, too. :)

Yes, it probably is a short form of 'darling', and so it can be used to anyone in a relatively informal context. I imagine it would be the sort of term of endearment a friendly middle-aged waitress might use to customers, both male and female.

Can you give an example sentence? I've never heard this term.

Did you see this written or just hear it? If you heard it, it might have been "doll", which is an informal term of endearment, but which can also used ironically. A common usage would be, "You're a doll." to thank a woman for doing something.