I think the lowercase "and" basically makes it act more like this: $csv and ($xls = $csv)

Yep. That's basically the entire reason that the and and or operators exist. and and or are low-precedence (making them useful for flow control), while && and || are high-precedence (making them useful for boolean logic). This is why, 99% of the time, open ... or die is correct and open ... || die is wrong.