use PDF::API2;
use DBI;
use strict;
use constant mm => 25.4/72;
use constant in =>    1/72;
use constant pt =>    1;


$pdf = PDF::API2->open('Build.pdf');

my %font = (
    Helvetica => {
        Bold   => $pdf->corefont('Helvetica-Bold',    -encoding => 'la
+tin1'),
 #      Roman  => $pdf->corefont('Helvetica',         -encoding => 'la
+tin1'),
 #      Italic => $pdf->corefont('Helvetica-Oblique', -encoding => 'la
+tin1'),
    },
    Times => {
 #      Bold   => $pdf->corefont('Times-Bold',        -encoding => 'la
+tin1'),
        Roman  => $pdf->corefont('Times',             -encoding => 'la
+tin1'),
 #      Italic => $pdf->corefont('Times-Italic',      -encoding => 'la
+tin1'),
    },
 );

my $page = $pdf->page;
 my $headline_text = $page->text;
 $headline_text->font( $font{'Helvetica'}{'Bold'}, 18/pt );
 $headline_text->fillcolor( 'white' );
 $headline_text->translate( 19/mm, 131/mm );
 $headline_text->text_right( 'this is my new stuff' );

$pdf->saveas("Build2.pdf");
[download]

It's your my $page = $pdf->page; that's creating the new page.

To put further content on an already existing page, simply use the method openpage instead, e.g. my $page = $pdf->openpage(1) for the first page -- see the section "PAGE METHODS" in the module's docs for the details.