sub append_n {
    my ($str, $array) = @_;
    my $vec = '';
    for (@$array) {
        my $ofs = 0;
        while ( ( my $idx = index $str, $_, $ofs ) > -1 ) {
          # Set bits at each matched location
          vec( $vec, $_, 1 ) = 1 for $idx .. $idx + length() - 1;
          $ofs = $idx + 1;
        }
    }
    # Count set bits
    vec( $vec, $_, 1 ) or substr( $str, $_, 1 ) = 'N'
      for 0 .. -1 + length $str;
    return $str;
}
__END__
GATTACGNNNNGCGCTCGNNNAACGGCA
GATTACGAGNNNNNNNCGTGTAANNNNN
NNNTACGAGTGGCGCTCGTGNNNNNNNN
NNNNNNNNNNGGNNNNNNNNNNNNGGNN