Just pick a reasonable value to test $ratio against (like maybe 1.0):

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1174957

use strict;
use warnings;
use Algorithm::Diff qw(traverse_sequences);

my $template = <<END;
The earliest conceptualization of human rights is credited to ideas ab
+out natural rights emanating from natural law. In particular, the iss
+ue of universal rights was introduced by the examination of extending
+ rights to indigenous peoples by Spanish clerics, such as Francisco d
+e Vitoria and Bartolomé de Las Casas. In the Valladolid debate, Juan 
+Ginés de Sepúlveda, who maintained an Aristotelian view of humanity a
+s divided into classes of different worth, argued with Las Casas, who
+ argued in favour of equal rights to freedom from slavery for all hum
+ans regardless of race or religion. 
END

my $copywitherrors = <<END;
The earliest conceptulization of human rights is credited to ideas abo
+ut rights emanating from natural law. In particular the issue of univ
+ersal rights was introduced by extending rights to indigenous peoples
+ by Spanish clerics, such as Francisco de Vitoria and Bartolomé de La
+s Casas. In the Valladolid debate, Juan Ginés de Sepúlveda, who maint
+ained an Aristotelian view of humanity as divided into clases of diff
+erent worth argued with Las Casas, who argued in favour of equal righ
+ts to freedom from slavery for all humans regardless of race.
END

my $somethingelse = <<END;
Although ideas of rights and liberty have existed in some form for muc
+h of human history, there is agreement that the earlier conceptions d
+o not closely resemble the modern conceptions of human rights. Accord
+ing to Jack Donnelly, in the ancient world, "traditional societies ty
+pically have had elaborate systems of duties... conceptions of justic
+e, political legitimacy, and human flourishing that sought to realize
+ human dignity, flourishing, or well-being entirely independent of hu
+man rights. These institutions and practices are alternative to, rath
+er than different formulations of, human rights".14 The history of hu
+man rights can be traced to past documents, particularly Constitution
+ of Medina (622), Al-Risalah al-Huquq (659-713), Magna Carta (1215), 
+the Twelve Articles of Memmingen (1525), the English Bill of Rights (
+1689), the French Declaration of the Rights of Man and of the Citizen
+ (1789), and the Bill of Rights in the United States Constitution (17
+91) 
END

sub compare
  {
  my ($old, $new) = @_;
  my $changes = 0;

  traverse_sequences( [ split //, $old ], [ split //, $new ],
    {
    DISCARD_A => sub { $changes++ },
    DISCARD_B => sub { $changes++ },
    } );
  return $changes;
  };

my $length = length $template;
print "length: $length\n";

my $answer = compare($template, $copywitherrors);
my $ratio = $answer / $length;
printf "copywitherrors: %d  ratio %.3f\n", $answer, $ratio;

$answer = compare($template, $somethingelse);
$ratio = $answer / $length;
printf "somethingelse: %d  ratio %.3f\n", $answer, $ratio;
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