Finishes section on CLI refs #114

Author: JJ

doc/Language/system.pod6

@@ -21,6 +21,14 @@ This will fail if the environment variable C<API_KEY> has not been defined previ
 
 Perl 6 has a better way to deal with command line arguments if they represent file names: the L<C<$*ARGFILES>/language/variables#index-entry-%24%2AARGFILES> dynamic variable.
 
+=begin code
+for $*ARGFILES.lines -> $l {
+    say "Long lines in {$*ARGFILES.path}"
+        if $l.chars > 72 ;
+}
+=end code
+
+You can run this program this way C<argfiles.p6 *.p6>, for instance, and it will print a file name every time it finds a line longer than 72 characters. C<$*ARGFILES> contains filehandles of all files described in the command lines- C<.lines> will read in turn one line from every one of them, changing the value of C<$*ARGFILES.path> every time a new handle is being processed. In general, it provides a very convenient API for scripts that deal with sets of files.
 
 =head1 Running programs synchronous and asynchronously