Boy, are you gonna be surprised to find out that you can replace most of your logic with...

... main code ...
$size = "${size}x$size" unless $size =~ /%$/;
...
sub shrink {
  my $name = shift;
  my $img = Image::Magick->new;
  $img->Read($name);
  $img->Mogrify('geometry' => $size);
  $img->Write($name);
}
[download]

And then let $size be either "50%" or "72" at the top there. The former will make a half-size picture. The latter will make the largest image that fits into 72x72, while keeping the proportions, as all your code attempts to do.

Yeah, the ImageMagick docs are a real pain to get into, but once you've got the good tricks, it's rather nice.

-- Randal L. Schwartz, Perl hacker