Do not forget, most of the JSON modules will look for and utilize the TO_JSON method implemented by any references' packages that are contained in the main reference you are attempting to serialize. It comes in handy and I wish I knew about this long ago. Note your error at the end mentions TO_JSON/FREEZE (didn't realize FREEZE was also considered).

To be clear, this means that in you must implement a TO_JSON yourself in the package of the reference contained in the overall reference you are serializing. hth.

Conversely, if package references or instances are getting unexpectedly serialized correctly, then look for a TO_JSON in the module .pm(s) themselves.