Yes, the warning is right; if you're assigning a single scalar value to a hash you probably don't want to be using hash slice semantics. If your code was failing when you did it "the right way", there's a bug elsewhere in the code. I couldn't run the code you provided because I don't know what your initial data set looks like, nor do I know what $hash{Airline}'s initial state is before the code you showed.

Perhaps you could follow up with a post in this same thread that demonstrates the problem in a self-contained script that we can actually run to see the behavior. Chances are good that as you attempt to boil it all down to a small test script of under 20 lines, you'll discover the bug yourself and be able to fix it.

But yes... the warning is accurate. You shouldn't have to assign a single value to a hash slice like that.