#!/usr/bin/perl

use strict;
use warnings;

my @data = <DATA>;

for ((my $i, local $_, my $next) = (0, @data[0, 1]);
		$i < @data;
		($_, $next) = ($next, $data[++$i + 1])) {
	$next and $next =~ /^\s/ and ($_, $next) = ($_ . $next, $data[++$i + 1]) and redo;

	# processing goes here
	print "#$i: $_";
}

__DATA__
keyword1 data1 data2 data3
keyword2 data1 data2 data3
    data4 data5
    data6
keyword1 data1 data2 data3 data4
keyword3 data1

##</code><code>##

#0: keyword1 data1 data2 data3
#3: keyword2 data1 data2 data3
    data4 data5
    data6
#4: keyword1 data1 data2 data3 data4
#5: keyword3 data1