I know what creates the additional pages, but what says "hey, write to me now buddy!"

That's what the openpage() method does. It will return the page object. From that you can add whatever you want to that page. This is how I usually do that:

my $current_page = $pdf->openpage($page_num);
$current_page->gfx->textlabel(
    100, # X coord
    200, # Y coord
    $font,
    $font_size,
    "Some text to write",
);
$current_page->update();
[download]