#!/usr/bin/perl
use warnings;
use 5.16.2;

$/ = "\n+";
while (<DATA>) {
	if ($_ =~ m/\+/) {
			s/\n\+//;
	}
	print;
}
say '';

__DATA__
xabcd a1
+b1
+b2
+b3
+b4
+b5 abcd
xcdef c1
+d1
+d2
+d3
+d4
+cdef