use PDF::Tk;
my $pdftk=PDF::Tk->new(pdftk=>'/usr/bin/pdftk');

...

# instead of filling @pdfLines from <POUT>
my $pdf_with_bg = ""; # to be filled by pdftk
my $pdf_in_memory;
{ local $/ = undef; $pdf_in_memory = <POUT>; } # whole file as one big string

$pdftk->call_pdftk(\$pdf_in_memory, \$pdf_with_bg, 'background', '/path/to/background.pdf');

print "Content-Type: application/pdf\n";
print "Content-Disposition: attachment; filename=record.pdf\n\n";
print $pdf_with_bg;