package User;

# ...

sub open_account {
 my ($self, $account_id) = @_;
 return User::Account->new($self, $account_id);
}

package Account;

sub new {
  my ($class, $user, $account_id) = @_;
  my $self = { user => $user, account_id = $account_id };
  bless $self, $class;
}

sub balance {
  my $self = shift;
  # now you have $self->{user} for the user object
  # and $self->{account_id}
}