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

1 : 1
12 : 12 
...
12345678 : 38165472 
123456789 : 381654729 
1234567890 : 3816547290