Ah! That's nice.

I condensed it into:

#!/usr/bin/perl

use strict;
use warnings;

# store 'A' and 'B' as a bitstream
# (bit groups are stored big endian!)
my @bits = (
  1, 0, 0, 0, 0, 0, 1, 0,   # A / 0x41 / 0b01000001
  0, 1, 0, 0, 0, 0, 1, 0,   # B / 0x42 / 0b01000010
  1, 1, 0, 0, 1, 0, 1       # incomplete byte
);

my $data  = pack "b*", join q{}, splice @bits, 0, -(@bits % 8);

printf "data: %s\n", $data;                # AB
printf "bits: %s\n", join ', ', @bits;     # 1, 1, 0, 0, 1, 0, 1
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Update

Both variations do not convert properly if there actually isn't an incomplete byte. This version works:

# variant a) using floor()
use POSIX;
$data = pack 'b*', join q{}, splice @bits, 0, POSIX::floor(@bits / 8) 
+* 8;

# variant b) using int
$data = pack 'b*', join q{}, splice @bits, 0, int(@bits / 8) * 8;
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