my $dir = "H:/vpn/";
my $out = "H:/vpn/vpnwarn.out";
my $old = "H:/vpn/vpnwarn.old";

if (-e $old) {
   unlink($old) || die ("Could not unlink: $!");
}
if (-e $out) {
  mv ($out, $old) || die ("Could not mv: $!");
}
# create and write new $out here