This isn't the job of Perl or CGI. It's the domain of the web browser to decide when and how to create new windows. There's no information in a normal HTML document that says "by the way, this is a new window now".

That said, there are a couple of answers. One is to change the link target -- if you're using a GET request -- to pop up a new window. Another option is to use JavaScript to open a new window.

Of course, that will depend on how the user has configured his browser and how well said browser supports JavaScript.