Your registration form has an 'action' attribute that sends the information filled in the form fields to your script, which then parses them.

In that script of yours you can do all sorts of validations, namelly to check if any value is present and if it is a valid email. If it isn't, you can send an error message back to the user and display the form again.

Look at the CGI module, if you haven't already, to see how you can capture the information being passed.