If I've read your code snippets correctly - that's not the way to call back to the CGI script from a form.

The -onClick parameter specifies a client-side (typically JavaScript) function which your browser will attempt to call when the user clicks the submit button. If you haven't defined such a JS function, nothing will happen.

You need to make the Submit button send the request back to your CGI script running on the web server. I don't know the syntax for this when using the CGI module, but I suspect you need to add some detail to the call to start_form, specifically an ACTION which will be the URI of your CGI script containing the registeruser() function.

P.S. Please put code in <code> tags to make it easier to read.