update: just saw the response to update CGI to properly handle application/xml. That may be a better idea than using CGI::Simple -- YMMMV.

Then unfortunately you cannot use HTTP::Server::Simple::CGI out of the box, you'll need to tell it to use something other than CGI. The docs show how you could use CGI::Simple.

my $server = SimpleSrv->new(8080);
   $server->cgi_class('CGI::Simple');
   $server->cgi_init(sub {
     require CGI::Simple;
   });
   $server->host('127.0.0.1');
   $server->run();
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