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

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

sub func {
   # Any changes to $_ will affect the caller's var.
...
      ...
   }
}

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