PDF::Writer / adding content

I am using Austin Z.'s PDF::Writer to create invoices, which works
pretty
well. Now I am facing the need to add some text to the first page of the
document later in the creation process, e.g. while I am already writing
on
page two.

In the manual I found the following information:

PDF::Writer#first_page
The first page created during startup, useful for adding something to it
later.

I just cannot figure out, how to add something later. Any ideas anyone ?

Thanks,
Holger von Ameln