I need to get the value stored in javascript variable (var) to perl variable.

First, JavaScript is being run client-side in the browser, while the Perl code is being run server-side by the webserver. So, to get the value of a JavaScript variable into a Perl variable, there's no way around somehow sending it to the server with an HTTP request.

There are many ways this could be achieved in theory (e.g. Ajax)... But before making your life unnecessarily complicated, I'd first think hard about whether you really need to do it that way — in particular, as the selected value of a menu widget (which is what you'd have in your var v here) is being sent to the server anyway, when you submit the form.   Just my 2 cents :)