No one has mentioned the CPAN modules that help solve this sort of problem: List::Compare, Set::Scalar, and Set::Array. If "elements in exactly 2 lists" was the only 'set' operation in a program, I would use jhourcle's second solution. If more than one 'set' operation is needed, the modules start to really pay off.

Set::Scalar has a flexible syntax, allowing for a dense mix of overloaded operators and chained methods. It can solve the OP problem quite tersely: 

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();

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List::Compare is better for a generalized form of the OP problem (find elements occurring exactly N times in X lists). In addition to the expected set functions (get_intersection, get_symmetric_difference, etc.), it provides are_members_which, which builds a hash like this: 

{
   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 ],
}

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The number of elements in each of those value arrays is equal to the number of lists where key is found. 
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,
);

[download]

In reply to Re: array comparison question by Util
in thread array comparison question by Anonymous Monk

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