my $stretch = 10;
my $mismatches = 1;

substr $reference_str, 0, 0, 'x' x (length($small_str) - 1);
$reference_str .= 'x' x (length($small_str) - 1);

for my $start_pos (0 .. length($reference_str) - 1 - $stretch) {
    my $substr = substr $reference_str, $start_pos, length($small_str) - 1;
    my $xor = $small_str ^ $substr;
    for my $inner_pos (0 .. length($xor) - $stretch) {
        if ($stretch - $mismatches <= substr($xor, $inner_pos, $stretch) =~ tr/\0//) {
            my $match = substr $reference_str, $start_pos + $inner_pos, $stretch;
            next unless -1 == index $match, 'x';

            say $start_pos + $inner_pos + 1 - length $small_str;
            say substr $small_str, $inner_pos, $stretch;
            say $match;
        }
    }
}