> actually it works undef is not cast to "" and hence less than empty string

No, it works because sort is stable and you declared the lucky case to be the input. Try again with 

use List::Util qw{ shuffle };
my @array = shuffle((undef) x 3, qw/c b a/, ("") x 3);

[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re^3: Sort undef by choroba
in thread Sort undef by Anonymous Monk

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