#!/usr/bin/perl

use strict;
use warnings;

my @a = qw( a b c );
my @b = qw( a b c d );
 
# Get the individual differences.
my @a_minus_b = setminus(\@a, \@b);
my @b_minus_a = setminus(\@b, \@a);
 
# merge then together and remove possible duplicates
my @symmetric_difference = uniq(@a_minus_b, @b_minus_a);
 
# Present our results.
print 'List A: ', join(', ', @a),
"\nList B: ", join(', ', @b),
"\nA \\ B: ", join(', ', @a_minus_b),
"\nB \\ A: ", join(', ', @b_minus_a),
"\nSymmetric difference: ", join(', ', @symmetric_difference), "\n";
 
 
# Takes two array references. Returns a list of elements in the first
# array that aren't in the second.
sub setminus
{
my ($a, $b) = @_;
 
# Convert $b to hash keys, so it's easier to search.
my %b;
@b{@$b} = ();
 
return grep !exists $b{$_}, @$a;
}
 
# take a list and return only uniq items
sub uniq
{
my %saw;
return grep !$saw{$_}++, @_;
}