I think PDF::Reuse will be just what you need. I had a look at it yesterday after
nite_man's recommendation. It looks like an excellent module, and has the very useful PDF::Reuse::Tutorial to accompany it. Example 15 of this tutorial appears to be very much along the lines of what you're describing. The only thing is that it uses absolute x/y positions for insertion, rather than searching and replacing a specified string, so it's no good for me.