Thank you for your reply, it was the same as my superviser suggested :) I have no idea how to get rid of it, yet. Here is the code for the rotate. To get the 'step edge' images correct, I have to rotate a larger image and then crop it to the right size.

sub rotate {
    my ($img, $path) = @_;
    my @degrees = qw(45 90 135 180 225 270 315);
    my $cropped_orig = crop_image($img);
    my %images = (0 => $cropped_orig);
    foreach (@degrees) {
        my $image = GD::Image->new(CANVAS, CANVAS);
        $image->copyRotated($img, 32, 32, 1, 1, 64, 64, $_);
        $image = crop_image($image);
        $images{$_} = $image;
    }
    return \%images;
}
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