my $username = $dbh->selectrow_array("
   SELECT username FROM tbl WHERE id = 9
");

##</code><code>##

my $username = $dbh->selectrow_array("
   SELECT * FROM tbl WHERE id = 9
");

##</code><code>##

my $username = $dbh->selectrow_array("
   SELECT username FROM tbl WHERE id < 9
");