Because the assignment copies the values, this also has the eff
+ect of
       turning call-by-reference into call-by-value.  Otherwise a func
+tion is
       free to do in-place modifications of @_ and change its caller's
+ values.

           upcase_in($v1, $v2);  # this changes $v1 and $v2
           sub upcase_in {
               for (@_) { tr/a-z/A-Z/ }
           }
[download]

Of course you can take a reference to any value in @_ , but you were asking for modifying the caller's value, if I get it right.

greets,
--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}