my @order;
my $sth_1 = $dbh->prepare( "SELECT id FROM TABLE WHERE previous IS NULL" ) or die $DBI->errstr;
my $sth_2 = $dbh->prepare( "SELECT id FROM TABLE WHERE previous IS ?" ) or die $DBI->errstr;
$sth_1->execute() or die $DBI->errstr;
if ( $sth_1->rows() > 0 ) {
    my ($id) = $sth_1->fetchrow_array();
    push @order, $id;
    $sth_2->execute( $id ) or die $DBI->errstr;
    while ( $sth_2->rows() > 0 ) {
       my ( $id ) = $sth_2->fetchrow_array();
       push @order, $id;
       $sth_2->execute( $id ) or die $DBI->errstr;
    }
}