#Takes a csv file in which fields may contain null (empty) values. For each null
#value, substitutes the last non-null value in that field. E.g. 
#
#   ABC,FGD,123,TEST
#   ,,124,TEST
#   ,,125,TEST2
#   FGD,HJY,123,DATA
#   HJY,YUI,458,DATA2
#   ,,852,DATA
#
#converts to
#
#   ABC,FGD,123,TEST
#   ABC,FGD,124,TEST
#   ABC,FGD,125,TEST2
#   FGD,HJY,123,DATA
#   HJY,YUI,458,DATA2
#   HJY,YUI,852,DATA
#
#

use strict;
use Text::ParseWords;

my (@line, @previous);
my $j;

while (<>) {
  @line = parse_line ",", 1, $_ ;
  for ( $j = 0; $j < @line; $j++ ) {
    $line[$j] = $previous[$j] unless $line[$j];
  }
  print join( ",", @line);
  @previous = @line;
}