@parts = split m{
  "            # if we match a quote
  (?{ ++$x })  # increment quote counter
  (?!)         # and fail
  |
  \s+          # or if we match whitespace
  (?(?{$x&1})  # if $x is odd
    (?!)       # fail
  )            # (otherwise succeed)
}x, q{A B "C D" E F"G H" I};