GrandFather knows best.

Note also that a statement like
    $_ = shift;
clobbers the contents of the  $_ default scalar. Because  $_ is global, this may cause Problems.

If you must use  $_ in this way, I would recommend the use of local within a statement or subroutine block:
    local $_ = shift;
or
    local ($_) = @_;
(but this still affects the value of  $_ as seen by any function called within that block after the point of local-ization!)