# more update: this is the tested version now!!

sub CountSubstrings { # $string1, $string2, $substr_length
    my $l = pop @_;
    my %found = ();
    my %match = ();
    my $first = 2;
    my $string1 = $_[0];
    for my $string ( @_ ) {
        $first --;
        my $ls = length( $string );
        my $limit = $ls - $l + 1;
        for ( my $i = 0; $i < $limit; $i++ ) {
            my $sbstr = substr( $string, $i, $l );
            $first or defined ( $match{$sbstr} ) && next();
            $found{ $sbstr }{ $string } ||= 1;
            $first and next;;
            defined ( $found{ $sbstr }{ $string1 } )
              and $match{ $sbstr } = 1;
        }
    }
    Count( keys %match );
}
sub Count { 1+$#_ };