sub func {
   # Any changes to $_ will affect the caller's var.
   local *_ = \$_[0];
   ...
}

##</code><code>##

sub func {
   my ($s) = $_[0];  # Safe.
   local *_ = \$s;
   ...
}

##</code><code>##

sub func {
   # Any changes to $_ will affect the caller's var.
   for ($_[0]) {
      ...
   }
}

##</code><code>##

sub func {
   for (my $s = $_[0]) {  # Safe.
      ...
   }
}