There's an article on ibm.com that discusses customizing pdf's via a middle ASCII postcript step and includes a small Perl application in the example. It's here at "Personalizing PDF files".

It's a fairly plain language description of the process. It's a shame there's no simple CPAN solution for filling in fields in a pre-existing pdf form and outputting the new file. Maybe I missed it, but I can't find one either. Maybe someone could concoct one that hid the PS middle step </hint>. :)

Also, Adobe has their FDF (File Document Format) which, as I understand it, is a text file format (constructed via CGI from some source, ie. html form) that can be served to a browser (which has Acrobat Reader installed), and the Reader will automatically retrieve and serve the applicable PDF form with the fields properly incorporated.

There's quite a bit of discussion (Google Groups)about using FDF via CGI indicating that several people have trouble understanding how to do it, but it certainly looks do-able. I haven't had occasion to do this, but it looks interesting so far.

HTH