my $sql  = "INSERT INTO authors (name, firstname) VALUES (?, ?);";
    $self->{st_insert_authors} = $self->{dbh}->prepare($sql);
    $sql= "UPDATE papers set year=? WHERE rero_id=?";
    $self->{st_update_papers} = $self->{dbh}->prepare($sql);
    $sql = "SELECT authorid FROM authors WHERE name=? AND firstname=?";
    $self->{st_select_authors} = $self->{dbh}->prepare($sql);
  ...