#!/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
        ];