You had asked for some examples of Perl modules that extend the core language. As was stated by @knighthacker, MooseX::Declare and Test::Class::Sugar are excellent examples:

https://metacpan.org/module/MooseX::Declare https://metacpan.org/module/Test::Class::Sugar

Just look at the SYNOPSIS section for example code. If you know some Perl, you will notice that the code would normally not be valid syntax. Yet simply by using a module, you can define classes (MooseX::Declare) or describe tests (Test::Class::Sugar) in a brand new way.