What you are doing:

my $gifdata = $img->gifanimbegin(1,0);
for (...) {
    ...
    $gifdata .= $gd->gifanimadd; # $gd?
    ...
}
$gifdata .= $img->gifanimend;
[download]

Just guessing, but I don't think you could add frames using a different GD object. You should probably draw the contents of $gd on top of $img in some way and then call $img->gifanimadd.