The only difference between a hidden field and a regular text box or other field is that it's hidden from the user.(*) It can still hold a value, assigned at runtime like above or even assigned via Javascript (like document.myform.somefield.value = "hello"). The value is passed in the POST or GET to the form receiver as any other field would be. It's just not directly visible to the user.
An aside: I invite you to read up on the usage of CGI.pm. It provides much easier ways if doing what you're doing, via methods like textfield(), hidden(), etc. (perldoc CGI will get you the CGI docs) There are also excellent tutorials here.
* And it's not truly hidden, anyone can view source, copy it insert their own values, post the form, so don't think of it as secret, just hidden.
cheers!
