#!/usr/bin/env perl

use strict;
use warnings;

my $in_plus_lines = 0;

while (<DATA>) {
    chomp;

    if (/^\+(.*)$/) {
        print "\n" unless $in_plus_lines;
        $in_plus_lines = 1;
        print $1;
    }
    else {
        $in_plus_lines = 0;
        print "\n" if $. > 1;
        print "$_";
    }
}

print "\n";

__DATA__
* asterisk_line_1
*
.dot_line
+ plus_line_1
+ plus_line_2
* asterisk_line_3
+ plus_line_3
+ plus_line_4
* asterisk_line_4
+ plus_line_5
+ plus_line_6
plain_text_line_1
+ plus_line_7
plain_text_line_2
+ plus_line_8
*
*asterisk_line_6
*asterisk_line_7
plain_text_line_3