#!/usr/bin/perl

my @arr = (["a","b","c"],
           ["a","b","d"]);

my ($r1, $r2) = @arr;
my ($m1, $m2);
for my $i (0..$#$r1) {
    if ($m1 and $m2) {
        print "$m2\n$m1\n$r1->[$i],$r2->[$i]\n";
    }
    $m2 = $m1;
    $m1 = $r1->[$i] eq $r2->[$i] ? $r1->[$i] : undef;
}

__END__
a
b
c,d

##</code><code>##

     (["a","c","a","b","c",...],
      ["a","d","a","b","d",...])