. . .
my $query = qq(
       with recursive cte (comment_ID, comment_post_ID, comment_author, comment_parent,
                           comment_date_gmt, comment_type, comment_content) as (
            select     comment_ID,
                       comment_post_ID,
                       comment_author,
                       comment_parent,
                       comment_date_gmt,
                       comment_type,
                       comment_content
            from       wp_comments
            where      comment_ID = ? AND comment_approved = 1
            union all
            select     p.comment_ID,
                       p.comment_post_ID,
                       p.comment_author,
                       p.comment_parent,
                       p.comment_date_gmt,
                       p.comment_type,
                       p.comment_content
            from       wp_comments p
            inner join cte
                       on p.comment_parent = cte.comment_ID
       )
       SELECT *
              FROM cte ORDER BY comment_date_gmt;
    );
    my $sth = $dbh->prepare($query);
    $sth->execute($id);

    while(my $row = $sth->fetchrow_hashref) {
        my $cid       = $row->{comment_ID};
        my $parent_id = $row->{comment_parent};
        my $post_id   = $row->{comment_post_ID};

        if ($parent_id eq 0) {
            push(@{$posts{$post_id}}, $cid);
        }
        
        $comments{$cid}->{comment_post_ID}  = $row->{comment_post_ID};
        $comments{$cid}->{comment_parent}   = $row->{comment_parent};
        $comments{$cid}->{comment_author}   = $row->{comment_author};
        $comments{$cid}->{comment_date_gmt} = $row->{comment_date_gmt};
        my $content = $row->{comment_content};
        $content =~ s|(\s*)\n(\s*)\n|$1<br />\n$2<br />\n|gm;
        $comments{$cid}->{comment_content}  = $content;
        
        push (@{$hierarchy{$parent_id}}, $cid); 
    }

    $sth->finish();

. . .