Now, this might be the most overobvious answer in the world, but anyway...

https is a bidirectional encrypted transfer method between your browser and webserver, that makes the data unreadable while in transit. You configure it in your webserver by adding a certificate to it. The encryption is then transparent for your content providers, ie. you use your cgi script as you would with http. No need for encryption in your perl script, it isn't even aware that the data is being encrypted.

So with https, you have data encryption. Now you need user authentication and authorization, and possible session management. There are several ways to accomplish that (ie. basic authentication realms, sessionid's in URL, cookies etc.)