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

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

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