Although I'm not 100% clear on what you want maybe the answer for you is to use the array index:

#!/usr/bin/env perl 
use strict;
use warnings;

my @array = (<DATA>);

my $i = 0;
while ($i <= $#array) {
    print "Line $i is $array[$i++]";
    while ($i <= $#array && $array[$i] =~ /^ /) {
        print "Line $i is a continuation: $array[$i++]";
    }
}

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