I'm not sure how the code above is relevant, as its doesnt use an IPv6 address/host. I'm able to send XML-RPC requests when connecting to an IPv4 address, as following :

my $host = "localhost";
my $port = 8011;
my $cli = RPC::XML::Client->new("http://$host:$port/RPC2");
my $cli_client = RPC::XML::request->new('AnaXmlRpcHandler.execute', ,R
+PC::XML::array->new(map {RPC::XML::string->new($_)} @arr));
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Have you been able to send XML-RPC requests to an ipv6 addresss?