Check out http://php.net/manual/en/reserved.variables.get.php:

An associative array of variables passed to the current script via the URL parameters.

So this is the same data that should be found in catalyst in $c->request->query_parameters->{field}; (catalyst experts please correct me if I'm wrong here).

And yes, the corresponding method to mix code and html with perl is templating, any templating modul will work.