while ( ($book_id) = $sth->fetchrow_array() ) {
    $booklist_1{$book_id} = 1;
}

##</code><code>##

while ( $book_id = $sth->fetchrow_arrayref() ) {
    $booklist_1{ $book_id->[0] } = 1;
}

##</code><code>##

while ( ($book_id) = $sth->fetchrow_array() ) {
    $booklist_1{$book_id} = {};
}