I think you get what you ask for:) XMLRPC supports arbitrary nested data structures. I have no experience with Frontier::Client, I assume you can format request and response by using the XMLRPC types array and struct. I think you can try something like:

my $result = $server->call('MyCall', {lowerBound => "one", upperBound 
+=> 'two'});
[download]

Cheers

Harry

UPDATE

Please ignore post, CountZero did not only beat me by at least half an hour, he also provided a working solution!