Try setting the quality in the Image::Magick constuctor, like this:

use constant IMAGE_QUALITY => 75;
my $img = Image::Magick->new(size => $size, quality => IMAGE_QUALITY);
$img->Read($image->{sourcedir}."/".$image_in);
$img->Write($tmp);
[download]

I have a program written like this that seems to work. :-)