$c1 = substr $s1, $i, 1;
my $c2 = length $s2 <= $i ? undef : substr $s2, $i, 1;
my $next = $i + 1 >= length($s1) ? $c1 : chr( ord($c1) + 1 );
$c1 = substr $s1, $i, 1;
my $c2 = length $s2 <= $i ? undef : substr $s2, $i, 1;
my $next = $i + 1 >= length($s1) ? $c1 : chr( ord($c1) + 1 );