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

my $grouping = q();
while (<DATA>) {
    my @columns = split;
    print "@columns $grouping\n";
    if (@columns > 2) {
        $grouping = $columns[1] if q() eq $grouping;
    } else {
        $grouping = q();
    }
}

__DATA__
REC1 DATAX DATA2 1
REC2 DATA3 DATA4 2
REC3 DATA5 DATA6 3
REC4 DATA7 DATA8 4
RECZ 5
REC1 DATAY DATA2 6
REC2 DATA3 DATA4 7
REC3 DATA5 DATA6 8
REC4 DATA7 DATA8 9
RECZ