sub bycreatetime
      {
             return $nodes{$a}{createtime}
                  - $nodes{$b}{createtime};
      }
      
      foreach $node (sort { bycreatetime } keys %nodes)
      {
            # ...
      }

##</code><code>##

      my (%sort_method) = (
            createtime => \&bycreatetime,
            createuser => \&bycreateuser, # For example
      );

      foreach $node (sort $sort_method{$sort_field} keys %nodes)
      {
            # ...
      }

##</code><code>##

      sub NumericalSort
      {
            my ($field) = @_;

            return sub {
                    return $nodes{$a}{$field}
                         - $nodes{$a}{$field};
            };
      }

      my (%sort_method) = (
            createtime => NumericalSort('createtime'),
      );

      foreach $node (sort $sort_method($sort_field) keys %nodes)
      {
            # ...
      }