I can answer the last part about the bit shift operator.
>> 12 takes the number to the left and shifts it 12 bits to the right.
<< 12 would shift it 12 bits to the left.
In octal there are 3 bits per character shown (is this still called a nibble? Or is that only 4 bits, I don't play in octal that often :)
So this will bitwise and $mode with 0170000 and then shift it down so that the bottom 4 zero's go away.
0170000 >> 12 = 017
Hope this helps!