It sounds to me like you're asking for some client-side behavior after a particular value is entered into a text field in Internet Explorer (or presumably whatever browser the end user chooses). Perl isn't generally used for client-side trickery. For that, you may be looking for Javascript, or some other client-side solution. Perl, as applied to web programming, is generally run on the server side as a CGI program. From the server side, there is not really anything you can do to manipulate text fields in a browser in real-time.

Web applications are usually broken down into two components: the server side programming, and the client side programming. A CGI application, possibly programmed in Perl, can output HTML or just about anything else, including Javascript, which is then executed on the client's browser (client side). But at that point, your question becomes a Javascript question, not Perl.