use strict;
use warnings;
my @array1 = ( 1, 2, 5,    9, 11, 12, 13 );
my @array2 = ( 1, 2, 5,       11, 12, 13 );
my @array3 = ( 1, 2, 5, 7, 9,     12, 13 );

use Set::Scalar;
my $x = Set::Scalar->new(@array1);
my $y = Set::Scalar->new(@array2);
my $z = Set::Scalar->new(@array3);

# Either way works.
#my $answer = $x->union($y,$z)
#           - $x->unique($y,$z)
#           - $x->intersection($y,$z);
my $answer = ($x + $y + $z)
           - ($x / $y / $z)
           - ($x * $y * $z);

my @only_two = $answer->elements();

##</code><code>##

{
   1 => [ 0, 1, 2 ],
   2 => [ 0, 1, 2 ],
   5 => [ 0, 1, 2 ],
   7 => [       2 ],
   9 => [ 0,    2 ],
  11 => [ 0, 1    ],
  12 => [ 0, 1, 2 ],
  13 => [ 0, 1, 2 ],
}

##</code><code>##

use List::Compare;
sub elements_found_in_exactly_n_lists {
  my ($n, @array_refs) = @_;
  my $lc = List::Compare->new(@array_refs);
  my @u = $lc->get_union();
  my %h = %{ $lc->are_members_which(\@u) };
  return grep { @{$h{$_}} == $n } keys %h;
}

my @only_two = elements_found_in_exactly_n_lists(
  2, \@array1, \@array2, \@array3,
);