my @array1 =( 1, 3, 6, 8);
my @array2 =( 1, 2, 3, 4, 4);
my @array3 =( 1, 2, 3, 5, 6);

my %present;
my @arrays = ( \@array1, \@array2, \@array3);
for my $i ( 0 ..  @arrays - 1) {
    $present{ $_  }{ $i } = 1 for @{ $arrays[ $i ]  };
}

my @only_two = grep { 2 ==  scalar keys %{$present{ $_ }}  } keys %present;

print "@only_two\n";