.. Tests many of the global variables defined within a perl directive

.. perl:: "SOURCE = $SOURCE"
.. perl:: "LINENO = $LINENO"
.. perl:: "DIRECTIVE = $DIRECTIVE"
.. perl:: "SUBSTITUTION = $SUBSTITUTION"
.. |subst-name| perl:: "SUBSTITUTION = $SUBSTITUTION"
.. perl:: "INCLUDES = " . join ' ',@INCLUDES
.. perl:: "TOP_FILE = $TOP_FILE"
