(?:(?<!\\)\")

I read it as doublequote which is not preceded by backslash

But what about an escaped backslash \\" or two \\\\" ... ?

I'd rather try something like (Untested pseudocode)

s/^(?:$escaped|$quoted|\S)*\K\s+/\n/g

and

$escaped = qr/\\./;

$quoted  = 
   qr/
     (['"])                  # start
     (?: $escaped | [^\1] )* # inside 
      \1                     # end, probably \g-1 better
    /x;
[download]

NB: I didn't cover the case of unclosed quotes, which is unclear anyway.

update

tested - fails - good night! :)

update

see Re: solution wanted for break-on-spaces (w/specifics) for "working" example