I just completed a section on code generation in the Modern Perl book draft. Per an interesting coincidence, Moose and Class::MOP reached 1.0 on the same day.

What's the coincidence? The final subsection of Code Generation discusses the use of Class::MOP for metaprogramming.

It's simple to explain code generation and metaprogramming in terms of string eval ; everyone who knows how to declare a class in Perl can figure out string quoting issues and the particular timing of BEGIN blocks and visibility and implicit closures when writing:

eval <<END_CLASS; package $class; use parent '$parent'; sub new { ... } sub get_name { ... } sub set_name { ... } END_CLASS;

... versus:

package My::Awesome::Subclass; use parent 'My::Awesome::Parent'; sub new { ... } sub get_name { ... } sub set_name { ... }

... but you end up having to deal with all of those issues, plus the drawbacks of string eval (it's slower than regular declaration, you don't get syntax highlighting or other IDE tools, it may leak memory in some versions of Perl 5, quoting can be a pain, parameterization is an exercise in templating systems rather than OO design, and there's no introspection or mechanism to modify the code after you've run it through eval ).

You also don't get guarantees that you've written your code correctly; you might have left out an important method or an important declaration, or you may manipulate @ISA in a messy or unfortunate way, or you may step all over an existing class elsewhere, or....

It's important to have the safety valve of string eval , at least so that experienced programmers can escape from the cozy confines of what the language itself has provided if they're doing brilliant things or if they're doing dirty things and have made the sufficient determination that the risk of getting caught for cheating is worth the value of getting things done quickly.

Yet Class::MOP for all of its perceived metacircular brain-twisting confusion (and there's really only one secret to learn to untwist all of that perceived complexity, unless you want to implement or bootstrap the system) gives you all of that power and more, and all of the missing correctness, and all of the syntax highlighting, and all of the "Hey, this is just Perl I don't have to quote or worry about timing or anything wild!" if you learn that one little secret: you're only manipulating objects.

Yes, the perceived syntactic weight of creating a class with Class::MOP is higher than eval "package Foo; \@ISA = 'Bar';"; , but you get introspection. You get interoperability. You get the benefit of a suite of thousands of tests and hundreds of real-world modules on the public CPAN alone ensuring that the system works. Just as Moose makes the right designs easy to choose, so too does using a formalized mechanism for metaprogramming make metaprogramming safer and easier.

(As well, anything which hides some of the gory details of Perl 5 implementation behind a nicer syntax than what Perl 5 provides in this area is an improvement on those terms alone.)