table:
child_node_id  parent_node_id  left_id  right_id
-------------  --------------  -------  --------
2              1             
3              1             
4              1             
5              1             
6              2             
7              2             
8              4             
9              5             
10             7             
9              10   

my $children = $dbh->prepare(qq{
  SELECT child_node_id FROM table WHERE parent_node_id = ?
});

my $setleft = $dbh->prepare(qq{
  UPDATE table SET left_id = ? WHERE child_node_id = ?
});

my $setright = $dbh->prepare(qq{
  UPDATE table SET right_id = ? WHERE child_node_id = ?
});

my $ctr = 1;
my $rootid = 1;

walktree($rootid);

sub walktree {
  my ($id) = @_;
  
  $setleft->execute($ctr++, $id);
  $children->execute($id);
  
  while(my ($subid) = $children->fetchrow_array) {
    walktree($subid);
  }
  
  $setright->execute($ctr++, $id);
}