I gave up using Image Magick in favour of Imager. If you have a look at the scale() function on the Imager::Transformations page you will see how easy it is to accomplish.

my $newimg = $img->scale(
    xpixels => 800,
    ypixels => 200,
    type => 'min',
);
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Obviously you only need to do this transform if the original image exceeds one of the dimensions.