Select Code to Download

use strict;
use warnings;
...

my $diff = relative_complement( list_2_hashref( @A ),
                                list_2_hashref( @B ) );

sub intersection {
  my %tally;
  $tally{ $_ }++ for map keys %$_, @_;
  return list_2_hashref( grep $tally{ $_ } == @_, keys %tally );
}

sub sym_diff {
  my %tally;
  $tally{ $_ }++ for map keys %$_, @_[0, 1];
  return list_2_hashref( grep $tally{ $_ } == 1, keys %tally );
}

my $A = list_2_hashref( qw( a b c ) );
my $B = list_2_hashref( qw( b c d ) );
...
symmetric difference A, B:
'a' => 1
'd' => 1

use strict;
use warnings;
...
relative complement A, B: (a)
intersection A, B, C: (c)
symmetric difference A, B: (a d)