use strict;
use warnings;
use Image::Magick;

my $pdf = Image::Magick->new;
$pdf->Read("foo.pdf");
$pdf->Annotate( font => 'Times',  pointsize => 12, fill => 'red', 
                text => "Wahoo!",  undercolor => 'white', 
                x => 40, y => 20 );
$pdf->Write("bar.pdf");