class X::Syntax::Augment::WithoutMonkeyTyping does X::Syntax { }

Compile time error thrown when augment is used without use MONKEY-TYPING.

Since augment is considered a rather unsafe and impolite action, you have to pre-declare your intent with the use MONKEY-TYPING; pragma.

If you don't do that, like here

augment class Int { };

you get the error

augment not allowed without 'use MONKEY-TYPING'