use warnings;
use strict;

## for parsing the CSV
use Text::CSV::Simple;
## for writing to excel
use Spreadsheet::WriteExcel;

my $datafile = ... the one you gave earlier ...

# Only capture feilds of interest
my $parser = Text::CSV::Simple->new;
$parser->want_fields(1, 3, 4, );
my @data = $parser->read_file($datafile);
## data is now read in (if you file is really big, you can do this on-the-fly too)
my %hash = (); ## for storing collated data
foreach @data{
  ## store terms acording to unique id
  push @{ $hash{$_->[0]}->[0] }, $_->[1]; ## disease term stored in an array
  push @{ $hash{$_->[0]}->[1] }, $_->[2]; ## MeSH term also stored in an array
}

## now print out data
# Create a new workbook called simple.xls and add a worksheet
my $workbook  = Spreadsheet::WriteExcel->new('simple.xls');
my $worksheet = $workbook->add_worksheet();

# The general syntax is write($row, $column, $token). Note that row and
# column are zero indexed
my $row = 0;
for my $key ( keys %hash ){
  $worksheet->write($row, 0, $key); ## id
  $worksheet->write($row, 1, ( join ', ', @{ $hash{$key}->[0] } )); ## diseases
  $worksheet->write($row, 2, ( join ', ', @{ $hash{$key}->[1] } )); ## MeSH

  ++$row; ## move to the next row
}
print "$0 completed : ".(scalar(localtime))."\n";