sub nCommonSubstrLenL2 {
    my( $haystack, $needle, $len ) = @_;
    ( $haystack, $needle ) = ( $needle, $haystack ) 
        if length( $haystack ) < length( $needle ); ## Added.
#    my $pattern = "A$len X" x int( length( $needle ) / $len );
    my $pattern = (" A$len" . 'X' x ( $len-1 )) x (length( $needle ) - $len +1);
    my $count = 0;
    my %possibles;
    for my $possible ( unpack $pattern, $needle ) {
        next if ++$possibles{ $possible } > 1;
        ++$count if 1+index $haystack, $possible;
    }
    return $count;
}