You might be interested in Image::Math::Constrain

Update:  Example:

#!/usr/bin/perl

use Image::Math::Constrain;

my $math = Image::Math::Constrain->new(800, 600);

my ($width, $height, $scale) = $math->constrain(1123, 1243);

printf "new width  = %.4f\n",   $width;
printf "new height = %.4f\n\n", $height;

printf "old aspect ratio = %.4f\n", 1123 / 1243;
printf "new aspect ratio = %.4f\n", $width / $height;  # same as old
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