No. Perl hashes have no order, so the sequence in which hash keys are serialized is more or less random.

Update: Note that Javascript does not define an order over the properties of objects either, so there is very little guarantee that a JSON document will have the keys in a specific order. It seems that ECMAscript 6 standardizes somewhat on "insertion order".

The JSON definition declares that

An object is an unordered set of name/value pairs.

So, for hashes, JSON does not specify an order of the keys.