I might be misunderstanding the question, but I believe I did something similar to what you might need a long time ago. I put all of my header code in to a separate file and called it when needed throughout my CGI.(multiple pages, forms, error pages, confirmation page ...) it acted more like a bread crumbing, but maybe this is what you are looking for? I second the use of Template::Toolkit, but this was writen before I was comforatble with it, and can an alernative if you are limited to what you can use.
in reply to CGI header
My 'header' displayHeader.pl:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- continue all of the junk you want to include in each page -->
From there, my CGI required the displayHeader file and called it like so:
The two functions above are entered based on the return value of a form validation subroutine (error checking form input).
print $cgi->p("Thank You");