#!/usr/bin/env perl

use strict;
use warnings;

{
    local $/ = 'keyword';

    while (<DATA>) {
        next if $. == 1;
        chomp;
        y/\n//d;
        print "$/$_\n";
    }
}

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

##</code><code>##

#!/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

##</code><code>##

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