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.