Your use of parentheses around some of the Bs in the OPed code and not others suggests that you might want to skip over the BBBBB group. If so:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $counter = '87654321';
 my @x =
   unpack 'a3 a3 a2 a a a a2 a3 x[a5] a11',
   unpack 'B*',
   pack   'H*', $counter
   ;
 dd \@x;
"
[100, "001", 11, 0, 1, 1, "00", 101, "01100100001"]
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