Doesn't seem that hard. The code below outputs the BUG_Comments_COMBINED.csv file, though it doesn't worry at all about the order in which it outputs lines. If that matters to you, it can be adjusted.

#!/usr/bin/perl -l
use strict;
use warnings;

my %comment; #comment for each bug id
<DATA>; #skip header line
while(<DATA>){
    chomp;
    #the 4 in the split below ignores all comma in the comment text
    my (undef, $bugid, $date, $body) = split(/,/, $_, 4); 
    $comment{$bugid} .= "$date $body ";
}

while (my ($id, $comment) = each %comment){
    print "$id, $comment";
}

__DATA__
CommentId, BugId, Date, CommentBody
1023,9555,03/06/2008,This is the body
1024,9555,03/07/2008,This is the body
1025,9555,03/08/2008,This is the body
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