sub permute {
    return if $_[0] && $_[0] / length($_[0]) != int ($_[0] / length($_[0]));
    if (!length($_[1])) { print $_[0]." "; return; }
    for (0..length($_[1])-1) {
         permute($_[0].substr($_[1],$_,1), substr($_[1],0,$_).substr($_[1],$_+1));
    }
}

##</code><code>##

1 : 1
12 : 12 
123 : 123 321 
123456 : 123654 321654 
12345678 : 38165472 
123456789 : 381654729 
1234567890 : 3816547290