I ran the crucial portion of your script on linux, and it worked fine. Maybe it's the version of ImageMagick on the server? There are other ways besides ->Thumbnail to resize, maybe Thumbnail isn't working on your server? You could try

$x->Resize(geometry=>'160x120');
#or
$x->Scale(geometry=>'160x120');
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#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $image = Image::Magick->new;

my $x = $image->Read('z.jpg');

$x = $image->Thumbnail(width=>240, height=>153);
  warn "$x" if "$x";

$x = $image->Write('zt.jpg');
  warn "$x" if "$x";
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