If you want to have data available for processing with Perl on the server, you will have to send it from the client ("browser") to the server.

For example, you could set a field in a <FORM> tag to a value and then submit that form.

Alternatively, one of the many Javascript libraries has ways of sending data using XMLHttpRequest to the server. For example with jQuery, there are the .get(), .post() and .html() methods that send data to the server and give the return value to Javascript code.