sub nCommonSubstrLenL {
    my( $haystack, $needle, $len ) = @_;
    ( $haystack, $needle ) = ( $needle, $haystack ) 
        if length( $haystack ) < length( $needle ); # Added
    my $count = 0;
    my %possibles;
    for my $ni ( 0 .. length( $needle ) - $len ) {
        my $possible = substr( $needle, $ni, $len );
        next if ++$possibles{ $possible } > 1;
        ++$count if 1+index $haystack, $possible;
    }
    return $count;
}