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;