The web by it's very nature is stateless. Someone can hit a web page and leave it loaded in their browser for days before doing something with it again. That's why if you are using just straight HTML and CGI you will have to create a new web page.

Naturally, people have devised ways to work around this. One example is using cookies to track state. Another is JavaScript, which brings us to pemungkah's node above, where he mentions AJAX. Ajax is what you are looking for, but that means even more stuff for you to learn. Personally, I would suggest you play with Perl CGI a bit more to get a better handle on how the technology works before moving on to more advanced topics.