The latter, @array = 15, is equivalent to the two operations "clear the array, store the value 15 in the first position." The former is "pre-extend or truncate the array so it has 15 elements." That's how things work with regular arrays. I've never used Bit::Vector::Array and can't advice on that. I've just told you how arrays in perl normally work. I use vec() for this kind of task normally.

As for "backwards" and "forwards," it appears the author has chosen to give you the data with the most significant bits first.