#!/usr/bin/perl -w

use strict;

my @IDS;
my $rowlen = 5;
my $last = [(0) x $rowlen]; # In case the first line has fields missing
while (<DATA>) {
    chomp;
    my @temp = split/\t/,$_;

    # set this element from the last if it's null
    (!defined($temp[$_]) or $temp[$_] eq '') and $temp[$_] = $last->[$_] 
        for 0 .. ($rowlen-1);

    push @IDS, $last = \@temp; # push saving a reference
}

print join("\t",@{$_}),"\n" for @IDS;

=pod output

% perl ddd.pl
0       0       0       0       0
1234    5       20021201        1       0
5678    0       20021202        1       0
5678    0       0       0       10
9120    10      20021211        0       0
6543    5       20021202        0       0
6543    0       0       0       5
6543    0       0       0       5
6543    0       0       0       5


=cut

__END__

1234	5	20021201	1	0
5678	0	20021202		
	0	0	0	10
9120	10	20021211	0	0
6543	5	20021202
	0	0	0	5
	0	0	0	5