Although you already have a working solution, I took this as a puzzle where the array

• has been filled before
• must not be shifted or modified otherwise
• shall be processed in a for-loop without a next statement

Here is my (not so serious) solution:

#!/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
[download]

which gives:

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