well, what are you trying to do?

The inner keysort on your code is equivalent to: 

@v=((grep { $_ <= $pivot } @$v), (grep { $_ > $pivot } @$v));

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is that what you intended?

I suppose that what you really want is to mimic the sort on the original post but numerically: sort first the elements > $pivot and then the rest.

Then, for your specific data, integers between -100 and 100, this works: 

my @v=nkeysort { $_<$pivot ? $_ + 200 : $_ } @$v;

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but if you know nothing about the numbers in $@v, then figuring a convenient sorting key can be quite difficult, it's easier to just use a grep/sort combination: 
my @v=((sort {$a<=>$b} (grep { $_ >= $pivot } @$v)),
       (sort {$a<=>$b} (grep { $_ < $pivot } @$v)));

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that inserted on the outer sort becomes: 
my @sorted = nkeysort {
  ( ( sort {$a<=>$b} (grep { $_ >= $pivot } @$_) ),
    ( sort {$a<=>$b} (grep { $_ <  $pivot } @$_) ) )[50]
} @data;

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BTW, note that for numeric keys you have to use nkeysort.

In reply to Re^2: use Sort::Key by salva
in thread Optimizing a sort function (wrap-around alpha) by SineSwiper

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