Oh? GD's docs are horrid, so I can see why you missed the following:

my $image = GD::Image->newFromPng($filename);

my ($height, $width) = $image->getBounds;

foreach my $x ( 0 .. $width ) {
    foreach my $y ( 0 .. $height ) {
        my ($index) = $image->getPixel($x,$y);
        my ($r,$g,$b) = $image->rgb($index);
        print "($x,$y) -> ($r,$g,$b)\n";
    }
}
[download]

That will print out the RGB value for every XY coordinate, row-first. There's a ton of other information you can extract. You just have to spend a few hours reading ALL the docs. It's well worth the effort.