my $sth = $dbh->prepare ("SELECT id, body FROM news");
$sth->execute ();
while (my ($news_id, $news_body) = $sth->fetchrow_array()) {
    my $sth = $dbh->prepare ("SELECT COUNT(id) FROM comments WHERE id = $news_id");
    $sth->execute (); # was this missing from your snippet accidentally?
    my ($comments) = $sth->fetchrow_array();
    print "$news_id, $news_body, $comments";
}