The following snippet is taken from Generate thumbnail and small overstruck images for web publication. $thumbX, $thumbY are the desired dimensions. The $portrait stuff allows the rectangle to be rotated to generate a landscape or portrait image depending on the aspect ratio of the original image. You probably want to omit that for your purposes.

  my ($height, $width) = $image->Get ('height', 'width');
  my $portrait = $height > $width;
  my ($x, $y) = ! $portrait ? ($thumbX, $thumbY) : ($thumbY, $thumbX);
  my $scaleX = $x / $width;
  my $scaleY = $y / $height;
  my $scale = $scaleX < $scaleY ? $scaleX : $scaleY;

  $thumb->Resize (width=>$width * $scale+0.5, height=>$height * $scale
++0.5);
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