my $eight_bit_piece = 255;
my $other_eight_bit_piece = 255;
my $sixteen_bit_piece = 8;

my $packaged_data = pack("CCn",
    $eight_bit_piece,
    $other_eight_bit_piece,
    $sixteen_bit_piece);
# Gives me 11111111 11111111 0000000000001000 as desired

##</code><code>##

my $piece_one_six_bits = 12;   # 001100
my $piece_two_six_bits = 12;   # 001100
my $piece_three_four_bits = 5; #   0101

# What now?

##</code><code>##

my $packaged_data = pack("n",
                          $piece_one_six_bits << (6 + 4)
                        | $piece_two_six_bits << 4
                        | $piece_three_four_bits
                        );