my @words;
  'foo bar baz baz burk | gah' =~ /
    (?{ local @_words })
    (?:
      (\w+)
      \s+
      (?{ local @_words = (@_words, $1) })
    )+
    (??{ quotemeta $_words[-1] })
    (?{ @words = @_words })
  /x;