yet another assertion keyword

 PerlX::Assert is a framework for embedding assertions in Perl code.
 Under normal circumstances, assertions are not checked; they are
 optimized away at compile time.
 .
 However if, at compile time, any of the following environment variables
 is true, assertions are checked, and if they fail, throw an exception.
 .
  - PERL_STRICT
  - AUTHOR_TESTING
  - EXTENDED_TESTING
  - RELEASE_TESTING
 .
 That is, assertions will only typically be checked when the test suite
 is being run on the authors' machine, or otherwise opted into.