#!/usr/bin/perl
use strict;
use warnings;
use AI::Genetic;
use Data::Dumper;

$|=1;
my $ga = AI::Genetic->new
  (
   -fitness => \&my_fitness,
   -type => 'rangevector'
  );

$ga->init([ [0,10], [0,10], [0,10], [0,10] ]);
$ga->sortIndividuals($ga->people());
# Note that using this instead of sortPopulation does NOT
# cause the same warnings:
# my @populace = sort {$a->score <=> $b->score} @{$ga->people()};

sub my_fitness {
  my $genes = shift;
  print ref $genes, "\n";

  my @sorted = sort {
    print Dumper($a,$b); sleep 1;
    $genes->[$a] <=> $genes->[$b]
  } 0..3;

  my $score = $sorted[1] + 2*$sorted[2] + 3*$sorted[3];

  print "DEBUG: score was $score\n";
  return $score;
}
[download]

1037% perl genetic.pl
ARRAY
$VAR1 = 0;
$VAR2 = 1;
$VAR1 = 2;
$VAR2 = 3;
$VAR1 = 1;
$VAR2 = 2;
$VAR1 = 1;
$VAR2 = 3;
$VAR1 = 0;
$VAR2 = 3;
DEBUG: score was 7
ARRAY
$VAR1 = undef;
$VAR2 = undef;
Use of uninitialized value in array element at genetic.pl line 27.
Use of uninitialized value in array element at genetic.pl line 27.
$VAR1 = undef;
$VAR2 = undef;
Use of uninitialized value in array element at genetic.pl line 27.
Use of uninitialized value in array element at genetic.pl line 27.
$VAR1 = undef;
$VAR2 = undef;
Use of uninitialized value in array element at genetic.pl line 27.
Use of uninitialized value in array element at genetic.pl line 27.
$VAR1 = undef;
$VAR2 = undef;
Use of uninitialized value in array element at genetic.pl line 27.
Use of uninitialized value in array element at genetic.pl line 27.
DEBUG: score was 14
   etc...
[download]

Update: It is an official bug: check out nested sort in perl-5.7.0 fails.

Right - the second time you try that sort, $a and $b are undef each time the block is evaluated. Furthermore, it appears that sort-within-sort only breaks if the inner sort is in a different package than the outer sort, as you'll notice that when I do the sort myself (inside main) there's no problem.

Also, see my other reply that exhibits this phenomenon without using AI::Genetic.

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/
[download]

As ugly as it seems, a workaround (not denying a likely perl bug here) is to use $::a and $::b in the inner sort. Which may mean patching AI::Genetic... although I don't know what that will do during non-embedded sorts.

#!/usr/bin/perl
use strict;
use warnings;

package yo;
use Data::Dumper;

sub my_f {
  my $genes = shift;

  my @order = (0,1,2,3);
  my @sorted = sort {
    local $SIG{__WARN__} = sub {print Dumper($a,$b); print @_; sleep 1
+;};
    $genes->[$a] <=> $genes->[$b]
  } @order;

  my $score = $sorted[1] + 2*$sorted[2] + 3*$sorted[3];

  print "DEBUG: score was $score\n";
  return $score;
}

package main;

$|=1;

my @populace = map { [ map {rand 100;} (1..4) ] } (1..100);
my @spopu = sort {yo::my_f($a) <=> yo::my_f($b)} @populace;
[download]

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/
[download]

The other mechanism is to use a subroutine with a ($$) prototype; then the arguments $_[0] and $_[1] will be used instead.

Maybe the second mechanism is useful for a workaround in either the inner loop or outer loop?