It might be simpler to do a first pass concatenating continuation lines via splice before your main processing.

johngg@shiraz:~/perl/Monks$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<__EOD__ or die $!;
keyword1 data1 data2 data3
keyword2 data1 data2 data3
    data4 data5
    data6
keyword1 data1 data2 data3 data4
keyword3 data1 
__EOD__

my @dataLines = <$inFH>;
chomp @dataLines;
close $inFH or die $!;

for my $idx ( reverse 0 .. $#dataLines )
{
    next if $dataLines[ $idx ] =~ m{^keyword};
    $dataLines[ $idx - 1 ] .= splice @dataLines, $idx, 1;
}

say for @dataLines;'
keyword1 data1 data2 data3
keyword2 data1 data2 data3    data4 data5    data6
keyword1 data1 data2 data3 data4
keyword3 data1
[download]

I hope this is of interest.