I am now not sure if I fixed the Experimental Keys error correctly or not as I am not understanding why the other stuff is breaking when fixing it.

Just FYI, my speculative (since I can't verify all these assertions) explanation of the chain of failures you're seeing is:

1. The Experimental keys on scalar is now forbidden ... error from the
       my @result = keys $results->{$ip};
   statement is a compile time error; no code is actually run.
2. Once the ... keys on scalar ... error is fixed, the code can compile and run and produces the Can't use an undefined value as a SCALAR reference ... run time error from the
       ${ $client->do_task(...) }
   expression, and execution stops.
3. If the ... undefined value as a SCALAR reference ... error is fixed, the code runs on until it hits the ... malformed JSON string ... error (which may be a warning rather than a fatal error; I haven't checked).

Again, please be aware that this explanation is largely speculative.
(Update: And Fletch's suggestion that the return value of each operation be checked to be as expected and has no object/class/module error status associated with it is an excellent BP IMHO. :)