Here's one way to do it (via an "ASCII-fied" bitstring):

$ perl -e 'printf "%x\n", unpack("v",pack("b12",scalar reverse unpack(
+"b12",pack("v",0x6bf))))'
fd6
[download]

or, if you don't mind the trailing zero (this operates on 16 bit instead of 12, i.e. 0x06bf -> 0xfd60)

$ perl -e 'printf "%x\n", unpack("n",pack("B*",unpack("b*",pack("v",0x
+6bf))))'
fd60
[download]