my $rows;
 if($id) {
   $sth = $dbh->prepare("SELECT id,name FROM category WHERE parent = '$id'");
   $sth->execute or die $dbh->errstr;
 } else {
   $sth = $dbh->prepare("SELECT id,name FROM category WHERE parent = '0'");
   $rows = $sth->execute or die $dbh->errstr;
 }

 if ( $rows + 0 ) {
    print qq~
 ...
~;
 }