($b, $c) = unpack("x6 A2 x5 A2", $a);   #forward 6, grab 2;backward 5, grab 2

And just to avoid another blind alley, the unpack template specifier for moving backward in a string is  'X' (upper case), not  'x' (lower case). See pack for a complete specification of pack/unpack template specifiers; also see the pack tutorial perlpacktut.