I have never seen this ...

$_ = "" => next unless defined;

... before. Could you explain how that works.

I'm not sure what part puzzles you, so...

• defined implicitly refers to $_.
• unless (...) { ... } can be written as ... unless ... if the block is just a simple statement.
• ... => ... is another way to write ... , ... (apart from the fact that => will autoquote any word left of it).
• In scalar context (ie the way used here) the comma (in either notation) evaluates its both arguments from left to right (and returns the value value of the last argument evaluated).

(You can find all details in perlsyn and perlop.)

In other words, that line is just one of the many variations of writing:

— Arien