File::CheckTree::validate() routine takes a single multi-line string
consisting of directives, each containing a file name plus a file test to try
on it. (The file test may also be a "cd", causing subsequent relative file
names to be interpreted relative to that directory.) After the file test you
may put || die to make it a fatal error if the file test fails. The default is
|| warn. The file test may optionally have a "!' prepended to test for the
opposite condition. If you do a cd and then list some relative file names, you
may want to indent them slightly for readability. If you supply your own die()
or warn() message, you can use $file to interpolate the file name.
