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

but it seems you didn't sue strictures, that's why I needed to add no warnings 'uninitialized';

> So you'd need to treat the 3 extra cases for $a, $b being undef and return -1,0,1 accordingly.

see cmp_undef2 for that solution

use strict;
use warnings;
use Data::Dump;

sub cmp_undef1 {
   return $a cmp $b 
     if defined $a and defined $b;
   no warnings 'uninitialized';
   return $b cmp $a;                    # invert order otherwise
}

sub cmp_undef2 {
   if (defined $a) {
      if (defined $b) {
         $a cmp $b;
      } else {
         -1
      }
   }
   else {
      if (defined $b) {
         1
      } else {
         0
      }
   }
}



my @array =( (undef)x 3, qw/c b a/, ("")x3);

my @result = sort cmp_undef1 @array;

dd \@result;

@result = sort cmp_undef2 @array;

dd \@result;
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output

["", "", "", "a", "b", "c", undef, undef, undef]
["", "", "", "a", "b", "c", undef, undef, undef]
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I don't want to elaborate on the sort SUBNAME LIST syntax see sort for details and other variations. (TIMTOWTDI I chose the shortest)

°) which is strange because

  DB<1> p "" eq undef
1
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