my $max = $#arr1;
$max = $#arr2 if $#arr2 < $max;
foreach my $i (0 .. $max) {
    push @add, $arr1[$i] unless $arr1[$i] == $arr2[$i];
}
if ($max < $#arr1) {
    push @add, $arr1[$_] for ($max .. $#arr1);
} elsif ($max < $#arr2) {
    push @add, $arr2[$_] for ($max .. $#arr2);
}