But if your keys are just numbers, then the array index can be the number you need. Note that in my example, I used undef as a place holder for array elements that did not have an entry. Unless your indices are not even remotely sequential or you have non-integer indices, an array will be faster, take less memory and will apply a useful constraint on your keys.

If there is a possibility that your arrays or hashes do not have equivalent keys, it is considered best practice to test prior to processing those values. Modifying the previously posted codes:

Hashes:

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

my %hash1 = ("1", "20",
             "2", "20",
             "4", "19",
             "5", "20",
             "6", "18",
             "7", "20");
my %hash2 = ("1", "19",
             "2", "20",
             "4", "16",
             "5", "19",
             "6", "20",
             "8", "20");

my %differences;
for my $key ( sort {$a <=> $b} keys %hash1) {
    unless (defined $hash2{$key}) {
        print "$key is in \%hash1 but not \%hash2\n";
        next;
    }
    
    $differences{$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
7 is in %hash1 but not %hash2

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

Arrays:

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

my @array1 = (undef,20,20,19,20,undef,18,20);
my @array2 = (undef,19,20,16,19,undef,20,undef,20);

my @differences;
my $top = $#array1 > $#array2 ? $#array1 : $#array2;
for my $i (1 .. $top) {
    if (defined $array1[$i] and not defined $array2[$i]) {
        print "$i is in \@array1 but not \@array2\n";
    }
    if (not defined $array1[$i] and defined $array2[$i]) {
        print "$i is in \@array2 but not \@array1\n";
    }
    next unless defined $array1[$i] and defined $array2[$i];
    
    $differences[$i] = $array1[$i] - $array2[$i];
    print "$i ($array1[$i] - $array2[$i]) = $differences[$i]\n";
}

print "\n", Dumper \@differences;
__END__
1 (20 - 19) = 1
2 (20 - 20) = 0
3 (19 - 16) = 3
4 (20 - 19) = 1
6 (18 - 20) = -2
7 is in @array1 but not @array2
8 is in @array2 but not @array1

$VAR1 = [
          undef,
          1,
          0,
          3,
          1,
          undef,
          -2
        ];
[download]

Note that in the array case, I will not miss that the second set has a value that the first one is missing.