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

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

my @differences;
for my $i (0 .. $#array1) {
    next unless defined $array1[$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
4 (19 - 16) = 3
5 (20 - 19) = 1
6 (18 - 20) = -2

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