There's actually a lot going on here under the hood, so let's step through it.

When you load Moose, a bunch of sugar functions are exported into your class, such as extends, has, with, and more. These functions are what you use to define your class. For example, you might define an attribute ...

As a convenience, Moose creates a new class type for your class. See the Moose::Manual::Types document to learn more about types.

It also creates a Moose::Meta::Class object for your class. This metaclass object is now available by calling a meta method on your class, for example Person->meta.

The metaclass object provides an introspection API for your class. It is also used by Moose itself under the hood to add attributes, define parent classes, and so on. In fact, all of Moose's sugar does the real work by calling methods on this metaclass object (and other meta API objects).

Note that each call to extends will reset your parents. For multiple inheritance you must provide all the parents at once, extends 'Foo', 'Bar'.

You can use Moose to extend a non-Moose parent. However, when you do this, you will inherit the parent class's constructor (assuming it is also called new). In that case, you will have to take care of initializing attributes manually, either in the parent's constructor, or in your subclass, and you will lose a lot of Moose magic.

See the MooseX::NonMoose module on CPAN if you're interested in extending non-Moose parent classes with Moose child classes.

Moose has a feature called "immutabilization" that you can use to greatly speed up your classes at runtime. However, using it incurs a cost when your class is first being loaded. When you make your class immutable you tell Moose that you will not be changing it in the future. You will not be adding any more attributes, methods, roles, etc.

This allows Moose to generate code specific to your class. In particular, it creates an "inline" constructor, making object construction much faster.

If you override new() in your class, then the immutabilization code will not be able to provide an optimized constructor for your class. Instead, you should use a BUILD() method, which will be called from the inlined constructor.

Alternately, if you really need to provide a different new(), you can also provide your own immutabilization method. Doing so requires extending the Moose metaclasses, and is well beyond the scope of this manual.