I second this. If i execute following code on a file, i get a small reduction in file size, but no loss in quality. If i execute the same code with quality '10', i get a much smaller file, but also a great loss in quality.

use Image::Magick;

my $image = Image::Magick->new;

my $x = $image->Read('test.jpg');

$image->Set(compression => 'JPEG');
$image->Set(quality => '100');

$x = $image->Write('out.jpg');
[download]

So it is normal to see a small decrease in file size.