Useful to pass through exec args

Example def from lisp user's group talk:

def titlecase : exec sbcl --script {
  (loop for line = (read-line *standard-input* nil nil)
        while line
        do (format t "~:(~a~)~%" line))
}
Document the exec "language"
Use $Config{perlpath} instead of $^X

This is the recommended way according to this page:

http://wiki.cpantesters.org/wiki/CPANAuthorNotes
Don't include MYMETA files in distribution

Fixes github issue #2