Looking at the title, this question does not seem to have much to do with Perl. I guess that you already know you can say document.forms[N].inputname.value='whatever'; to populate a form field using Javascript.

I suspect, however, that you have omitted to mention that the desired value should be somehow relevant to your application, in which case you would be well advised to check out HTML::FillInForm from the CPAN.

Update: Nothing at all to do with Perl, then, poster has incorrect Javascript syntax.