return unless ref($hashref) eq 'HASH';

##</code><code>##

return unless keys %$hashref;

##</code><code>##

sub treeify {
    my ($sth) = @_;

    my %root;
    my %children;

    while (my $row = $sth->fetch()) {
        my ($id, $parent_id) = @$row;

        my $parent = ( defined($parent_id)
            ? $children{$parent_id} ||= {}
            : \%root
        );

        $parent->{$id} = $children{$id} ||= {};
    }

    return \%root;
}

##</code><code>##

{
    my $sponge = DBI->connect(
        'dbi:Sponge:', '', '',
        { RaiseError => 1 }
    );

    my $sth = $sponge->prepare(
        'SELECT id, parent_id FROM Table',
        {
            NAME => [qw( id parent_id )],
            rows => [
                [ a => undef ],
                [ b => 'a'   ],
                [ d => 'c'   ],
                [ c => 'b'   ],
                [ e => 'a'   ],
            ],
        }
    );

    my $tree = treeify($sth);

    use Data::Dumper;
    print Dumper $tree;
}

##</code><code>##

$VAR1 = {
          'a' => {
                   'e' => {},
                   'b' => {
                            'c' => {
                                     'd' => {}
                                   }
                          }
                 }
        };