## extracted and simplified from a bit of working code I have
my $fname = "/path/to/your/file.jpg";
my $text = "What you want to say.";

open (IMG, "<$fname") or die;
my $size = -s $fname;
my $data; 
read IMG, $data, $size;
close IMG;
die "blah" unless $data;

use GD;
# my $itype = image_type(getType_bymagic($data));
# die unless $itype eq "Jpeg";
# my $method = "newFrom${itype}Data";
my $method = "newFromJpegData";
my $i = GD::Image->$method($data, 1);
my $fg = $i->colorAllocate(102,102,102);

$i->string(gdMediumBoldFont,7,7,$text,$fg);
return \$i->png;