use GD;

# read the image
my $gdimg = GD::Image->newFromPng("my_image.png");

# we need a color for our text
my $green = $gdimg->colorAllocate(0,255,0);

# write the string "TEST!" to the image
$gdimg->string(gdMediumBoldFont, 5, 5, "TEST!", $green);

# get the new image
my $data = $gdimg->png;

# write it out
binmode STDOUT;
print $data;