Actually Class::MOP does not provide for a (direct) way to add methods to an instance, it mostly deals with just classes. And with Moose, there is no way to add a single method on a per-instance basis really either, you do it with runtime Roles instead, like this:

{
    package My::Class;
    use Moose;

    sub hello { "Hello" }

    package My::Role;
    use Moose::Role;

    sub foo { "Foo" }
}

my $obj_1 = My::Class->new;
my $obj_2 = My::Class->new;

My::Role->meta->apply($obj_1);

print $obj_1->foo; # print "Foo"
print $obj_2->foo; # dies with a method not found error
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Also Class::Trait does this same type of thing using Traits instead of Roles.

Ruby accomplishes this type of thing with "Eigenclasses" which were in the Perl 6 metamodel at one time, but were not backported to Moose (because they tended to make simple metaclass things overly complex). The Ruby idiom of:

class << obj
  def new_method
   "fweeee"
  end
end
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roughly translates into this:

obj.add_singleton_method(lambda { "fweee" })
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where add_singleton_method is just a method in Object. Something like this could be added to Moose without too much trouble at all really (no AUTOLOAD tricks needed either).