"Why => in split /$field_separator/ => $record?"

Beyond being a preferred style, it has no particular significance.

"To my eye that says paired item, but there it's just a plain ol' comma and would be clearer as: ..."

I really can't believe you're having any difficulty with the clarity of that code; however, if you are, think of it in terms of this pairing:

how to split => what to split

With regards to the while condition, my intention was to point out the confusion between the filehandle and the record. I thought I'd made that fairly clear with my opening sentence. I didn't consider a more in-depth discussion of how <$filehandle> works to be useful or helpful in this context.

[To waytoperl: If you're wondering about this use of defined, it's explained in more detail in perlop: I/O Operators.]