$x = "\x55\xAA\x55\xAA";;

substr( $x, $_, 1 ) = chr( ord( substr( $x, $_, 1 )) << 1 ) for 0 .. 3; 

print unpack 'b*', $x;;
Character in 'b' format wrapped in unpack at (eval 34) line 1, <STDIN> line 26.
Character in 'b' format wrapped in unpack at (eval 34) line 1, <STDIN> line 26.
01010101001010100101010100101010