package main;                             
use X;                                    
use SORT;                                 
my $p = new X(10);                           
my$q = new X(20);                           
my $r = new X(15);                           
foreach my $obj (sort by_id ($p,$q,$r)){  
        print $obj->id;                   
}                                         
                                          
                                          
package X;                
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(id);    
                
sub new {                                 
  my $class =shift;                       
  my $self = { _id => shift };            
  bless $self, $class;                    
}                                         
sub id {                                  
  my $self = shift;                       
  return $self->{_id};                    
}                                         
1;                                        
package SORT;                             
use X;                                    
require Exporter;                         
@ISA = qw(Exporter);                      
@EXPORT = qw(by_id);                      
sub by_id {                               
   $a->id <=> $b->id;                     
}                                         
1;