Inheritance is for "is a" relationships. For instance, a BMW "is a" SportsCar, so BMW could inherit from SportsCar. Now let's try it with your class: An Account "is a" User. That isn't true, so Account would not inherit from User.

"Composition" is for "has a" relationships. For instance, a User "has a(n)" Account. In your case, composition would involve assigning an Account object to a User variable. If your User object uses a hash internally, then one key in the hash could be called ACCOUNTS, and its value could be a reference to an array. The array would contain various Account objects.

package User;

sub new {
  my ($class, $username) = @_;
  my $self =(username=> $username, accounts=>{} , # No accounts for a 
+new user
             server=> {Some server connection info} 
   );
  bless $self, $class;
}

sub open_account {
 my ($self, $account_id) = @_;
 return $self->accounts->{$account_id} = User::Account->new($self, $ac
+count_id);
}
# accessor for accounts..

sub get_account{
  my ($self,account_id) = @_;
  return $self->accounts->{account_id};
}
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