You can use Image::Thumbnail (which uses Image Magick) like so:

#!/usr/bin/perl
use strict;
use warnings;
use Image::Thumbnail;
                                                                      
+                                                            
my $image_name = 'picture.jpg';
my $output_image_name = 'picture_thumb.jpg';
                                                                      
+                                                            
my $t = new Image::Thumbnail(
  size       => 80,
  create     => 1,
  inputpath  => $image_name,
  outputpath => $output_image_name
);
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Update: Oops, after drinking my morning cup I noticed the question wasn't concerning the actual image size.