I would have posted a similar approach to induce probable column boundaries and then build a template string for unpack based on file data, but I had forgotten about the "string-OR" trick, so my code would end up much longer (and obviously was taking much longer to write).

Luckily for me, I rechecked the thread before getting to the tricky part in my approach (collapsing an array of character offsets into a pack template), and saw that there was a better way to do it (better in the sense of getting it done quicker with less code, as proven by the relative speed of your post).

++!