|| 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 filenames, 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 filename.
Filetests may be bunched: ``-rwx'' tests for all of -r, -w, and -x.
Only the first failed test of the bunch will produce a warning.
The routine returns the number of warnings issued.