First things first. This part right here:

my %paramfields = [{
                    'this'          =>  'thisvalue',
                    'that'          =>  'thatvalue',
                    'something'     =>  'somevalue'
                    }];
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That is pretty messed up. You are assigning, to a hash table, a single value that is an array reference, whose first (and only) element is a hash reference. This array reference would be the key, but there is no value to go with it. I suspect that you meant to create an array @paramfields that has all the parameters you plan to pass to the call.

Based on what you coded, the Frontier client did in fact encode it correctly.

And just because it's me, I feel obligated to promote RPC::XML, which I feel is more actively maintained than Frontier (though I haven't been very active these past few months).

--rjray