# pass in a file name, a file handle or nothing
# returns a unique string that changes when the file changes
sub FileMeta {
    if (my $fh = shift) {
        my ($dev, $inode, $size, $mtime) = (stat($fh))[0,1,7,9];
        return "$size:$dev;$inode:$mtime";
    }
    else {
        my ($dev, $inode, $size, $mtime) = (stat(_))[0,1,7,9];
        return "$size:$dev;$inode:$mtime";
    }
}