This sample should so what you want.

use PDF::API2;
use strict;

my $new = PDF::API2->new; # new doc
my $pdf = PDF::API2->open("testdata.pdf"); # existing foreground doc

my $page = $new->page;
my $bg = $page->gfx;
my $img = $new->image_jpeg ("test.jpg"); #Bg image
$bg->image($img, 0, 0, 612, 792); # locate in lower right, 8.5x11 pape
+r

my $form = $new->importPageIntoForm($pdf,1); #get page 1 from existing
+ doc
$bg->formimage($form,0,0,1);

$new->saveas( "try2.pdf");
[download]