create a buffer for the value in question. if the value is null, use the buffered version. otherwise, modify the buffer to the current value.

there are edge cases i haven't dealt with, but you weren't that specific about functionality in that space. in any case, this should get you in the right direction.

#!/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
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~Particle *accelerates*