##################################################
# toHash() in base class provides an empty hashref
##################################################
{
  package Base;
  use Moose;

  sub toHash { return {} }
}

#############################################
# In a role, specific attributes are going to
# be stored in hashref provided by base class
#############################################
{
  package FooRole;
  use Moose::Role;
  
  has foo => (is=>'rw', isa=>'Str');
  has bar => (is=>'rw', isa=>'Str');

  around toHash => sub {
    my($orig,$self) = @_;
    my $hash = $self->$orig();

    # I only want store foo, but not bar
    $hash->{foo} = $self->foo;
    return $hash;
  };
}

###################################################
# We may have many other roles that provide further
# modifications to the function.
# And those roles may be assembled in arbitrary
# pattern, which provides flexibility.
###################################################
{
  package Foo;
  use Moose;
  extends 'Base';
  with 'FooRole';
}

my $foo = Foo->new(
  foo => 'foo_value',
  bar => 'bar_value'
);

my $data = $foo->toHash # produces { foo => 'foo_value' }