# note: $fallback is \&File::Copy::copy() or \&File::Copy::cp()
    return 1 if rename $from, $to;
 
    # Did rename return an error even though it succeeded, because $to
    # is on a remote NFS file system, and NFS lost the server's ack?
    return 1 if defined($fromsz) && !-e $from &&           # $from disappeared
                (($tosz2,$tomt2) = (stat($to))[7,9]) &&    # $to's there
                  ((!defined $tosz1) ||                    #  not before or
                   ($tosz1 != $tosz2 or $tomt1 != $tomt2)) &&  #   was changed
                $tosz2 == $fromsz;                         # it's all there

    ($tosz1,$tomt1) = (stat($to))[7,9];  # just in case rename did something
    {
        local $@;
        eval {
            local $SIG{__DIE__};
            $fallback->($from,$to) or die;
            my($atime, $mtime) = (stat($from))[8,9];
            utime($atime, $mtime, $to);
            unlink($from)   or die;
        };
        return 1 unless $@;
    }
    ($sts,$ossts) = ($! + 0, $^E + 0);
 
    ($tosz2,$tomt2) = ((stat($to))[7,9],0,0) if defined $tomt1;
    unlink($to) if !defined($tomt1) or $tomt1 != $tomt2 or $tosz1 != $tosz2;
    ($!,$^E) = ($sts,$ossts);
    return 0;