#!/usr/bin/perl
use strict;
use warnings;
$|++;

my $buffer;
while( my $record = <DATA> )
{
	my @items = split /\t/, $record, 5;
	'' ne $items[0]
		? $buffer = $items[0]
		: $items[0] = $buffer;
	print join "\t", @items;
}

=pod

prints:
1234    5       20021201        1       0
5678    0       20021202        0       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

=cut

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


##</code><code>##

.