I've found that directly inserting text into a PDF can be fraught with danger. I suspect that the text/graphics state has been left in an altered state; I.E. its undergone rotation or translation etc.
I've found it more robust to recreate each page in the PDF and import each page as a form element. Then output any new text or graphics.
#!/usr/bin/perl
use warnings; use strict;
use PDF::API2;
my $file = shift (@ARGV);
my $pdf_in = PDF::API2->open($file);
my $pdf = PDF::API2->new;
my $pagenum = 1;
my $page_in = $pdf_in->openpage($pagenum);
my $page = $pdf->page(0);
$page->mediabox($page_in->get_mediabox);
$page->trimbox($page_in->get_trimbox);
my $gfx = $page->gfx;
my $xo = $pdf->importPageIntoForm($pdf_in, $pagenum);
$gfx->formimage($xo,
0, 0, # x y
1); # scale
my $font = $pdf->corefont('Helvetica-Bold');
my $text = $page->text();
$text->font($font,10); #10-point font size
$text->translate(15,770); #top left corner.. 0,0 origin starts in the
+bottom left corner
$text->text('blah blah sample text');
$pdf->saveas($file);