#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %hash1 = ("1", "20",
             "2", "20",
             "4", "19",
             "5", "20",
             "6", "18");
my %hash2 = ("1", "19",
             "2", "20",
             "4", "16",
             "5", "19",
             "6", "20");

my %differences;
for my $key ( sort {$a <=> $b} keys %hash1) {
    $differences{$key} = $hash1{$key} - $hash2{$key};
    print "$key ($hash1{$key} - $hash2{$key}) = $differences{$key}\n";
}

print "\n", Dumper \%differences;
__END__
1 (20 - 19) = 1
2 (20 - 20) = 0
4 (19 - 16) = 3
5 (20 - 19) = 1
6 (18 - 20) = -2

$VAR1 = {
          '6' => -2,
          '4' => 3,
          '1' => 1,
          '2' => 0,
          '5' => 1
        };