This version should do better (update: made more idiomatic/terse, it should really do aspect preservation to make a better image instead of translating an unknown aspect to a square)–

use strictures;
use GD;
use Path::Tiny;

GD::Image->trueColor(1);

my $original_image = GD::Image->new( shift || die "Give an image!\n" )
+;
my $new_image = GD::Image->new(100, 100);

$new_image->copyResampled( $original_image,
                           0, 0, 0, 0,
                           $new_image->getBounds,
                           $original_image->getBounds );

path("new.jpg")->spew_raw($new_image->jpeg);
[download]

The changes that make it “right” are GD::Image->trueColor(1) and $newimage->copyResampled instead of copyResized.