self_url method returns a string containing the complete
URL of the script being called; as seen in the previous section, this is very
useful for combo forms.
The user_agent method returns a string identifying the type of
browser which called your CGI script.
If you pass user_agent an argument, it will use it as a regular expression, and
return the value of a pattern match of the argument with the value of the user agent string.
The remote_host
method returns the hostname or IP address of the host under which the browser is
running. The referer method returns the URL of the page the browser was viewing
prior to invoking the CGI script. Other similar methods are available; see the
CGI module documentation.