I'm only learning Korean myself but I know 아냐 is no and 헐 sounds a bit like the english word hell, so maybe its a take on the English phrase 'Hell no!'..Im curious myself now lol..native speakers please enlighten us!

I like Elizabeth's analogy. 헐 is used in casual writing to represent a sound that people make when they are slightly surprised, 헐 아냐 means something like "That's a bit surprising, but it's nothing important."

elizabeth is right 아냐 means no, but 헐 is a nonsense word. So this sentence makes no sense