Figuring out how to keep a scaled image proportional shouldn't be too hard.

my $new_width = $orig_width;
my $new_height = $orig_height;

if ( $new_width > $max_width ) {
    $new_height = int( $orig_height / ( $orig_width / $max_width ) );
    $new_width = $max_width;
}
if ( $new_height > $max_height ) {
    $new_width = int( $orig_width / ( $orig_height / $max_height ) );
    $new_height = $max_height;
}
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I wouldn't be surprised if Image::Magick has a way to do this for you.