Which part of it is confusing?

I suspect you wonder why $x is undefined.

The for loop sets $_, but does not localize it. readZip() reads into $_, and the last line read is undef. That's why the while loop terminates.

for loops alias the control variable to the values iterated -- and readZip() modifies $_, which is an alias for $x within the loop. When the loop ends, $x still holds the last value assigned to $_ within the loop: undef.

local $_; at the start of readZip() would fix this. (You also don't need the prototype.)

Update: Removed some very unclear text, thanks to ikegami.