In my opinion, Javascript is best used for glue to hold together a web application, and for minor field validation (if at all). The load time of a page, if written in javascript coompared with perl/cgi, may increase, with the added disadvantage that the spinning globe (or whatever the browser uses to indicate waiting for a request) will stop while the javascript is rendering the page.

You will, of course, eliminate the perl compile-time delay :), which is less of an issue if your server has mod_perl or its ilk.

In terms of its use as glue, Javascript may be used to tie form fields together in a coherent way, such as disabling a set of controls depending upon the state of a checkbox or radio button, etc.

It also potentially has uses in rendering very large tables, since the download time can be shorter if the data is in javascript arrays than in generated HTML. This last use is controversial, since it obviously won't work if the browser has Javascript disabled.