If you are talking about the content inside the [..], then I'd agree that it contains an AOH.

From my perspective, the outermost
'main_data' => [...
indicates that it is either a hash or hashref. Of course, the programmer could be misusing the "=>" instead of a comma, making the outermost element an array, but that is marginally perverse.

So. in the absence of information to the contrary, I'll stick with my interpretation of HOAOH.