As we all know $#boo returns the last index of array @boo.
It is clear why we have the prefixes '$' and '@' ('$' is like the first
letter of the word 'scalar' and the '@' is like the first letter of the word
But is it unclear why there is '#' after the dollar sign. I've checked out
the perl v 1.0 and in the man page there is such text:
> you may find the length of array @days by evaluating "$#days", as in csh.
> [Actually, it's not the length of the array, it's the subscript of the last
> element, since there is (ordinarily) a 0th element.]
So the answer why the number of the last index is $#boo is somewhere is csh.