To do something on the client (the browser) side, you have to use JavaScript.

If you want to simply avoid the user having to hit the "submit" button, you can

<select onChange="document.form.submit();">
[download]

and then things will be sent to the server where you can use Perl to do more stuff with the values.

Otoh, if you want to do more magic on the client, you can

<select onChange="domagic();">
[download]

and put your magic logic in the JavaScript function in domagic.

Update: The node should be retitled. It currently says "Re: Cgi and Perl : how to use data once people selects the checkbox field?", but the OP is showing the select field; in other words, it could be "how to use data once people choose a value in the select field." Javascript methods for dealing with select widgets are different from those dealing with checkboxes.