s/(\s+)/$1 eq "\n\n" ? $1 : " "/eg;

##</code><code>##

($_ = reverse) =~ s{
  (?:
    [\r\t\f ]+          # non-\n whitespace
    |                   # OR
    (?<!\n)             # not preceded by a \n
    \n                  # match a \n
    (?=                 # that's followed by...
      (?:\n\n)* (?!\n)  # an even number of \n's
    )
  )+                    # one or more times
}{ }xg;                 # turn it into a single space
$_ = reverse;