use strict;

use constant NULL_VAL => -999.9;

my @data = ();

while (<DATA>) {
	chomp;
	s/^\s+|\s+//g;
	next if $_ eq '';

	push @data, [split ','];
}

my %last_good = map {$_ => NULL_VAL} (1..$#{$data[0]});

# Go in reverse to fill in null data
for (my $i=$#data; $i>=0; $i--) {
	# Validate each column
	for my $j (1..$#{$data[$i]}) {
		if ($data[$i][$j] == NULL_VAL) {
			$data[$i][$j] = $last_good{$j};
		} else {
			$last_good{$j} = $data[$i][$j];
		}
	}
}

foreach (@data) {
	print join(',', @$_) . "\n";
}

# time,col1,col2
# (-999.9 out-of-range to indicate missing
# and distinguish from true zeroes)
__DATA__
0.01,-999.9,1
0.02,-999.9,-999.9
0.03,-999.9,3
0.04,2,-999.9
0.05,-999.9,-999.9
0.06,5,4