#!/usr/bin/env perl

use strict;
use warnings;

my $multiline = '';

while (<DATA>) {
    chomp;

    if (0 == index $_, ' ') {
        $multiline .= $_;
    }
    else {
        print "$multiline\n" if length $multiline;
        $multiline = $_;
    }
}

print "$multiline\n";

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