Try this version:

use warnings;
use DBI;
use File::Basename;

my $dir   = '.';
my $file  = 'noname.csv';
my $table = (fileparse($file,'.csv'))[0];
my $cols  = [qw(Username Email Posts Timestamp)];
my $sep   = ',';

open outFile, '>', $file;
print outFile <<SampleData;
Username, Email, Posts, Timestamp
ed,ed\@there,11,10:23:01
bill,bill\@there,5,10:23:02
SampleData
close outFile;

my $dbh = DBI->connect(
   "DBI:CSV:f_dir=$dir;csv_eol=\n;csv_sep_char=$sep;",
   {RaiseError=>1},);

$dbh->{csv_tables}->{$table} =
  {
  file      => $file,
  col_names => $cols,
  };

my $sth = $dbh->prepare("SELECT Username, Email, Posts, Timestamp FROM
+ noname");
$sth->execute() or die "Cannot execute: " . $sth->errstr();
my %newPosts;

while ((my $Username, my $Email, my $Posts, my $Timestamp) = $sth->fet
+chrow_array ())
  {
  next if ! defined $Timestamp;
  print "$Username, $Email, $Posts, $Timestamp\n";
  $newPosts{$Username} = ++$Posts;
  }

for my $Username (keys %newPosts)
  {
  my $Posts = $newPosts{$Username};
  $sth = $dbh->prepare("UPDATE noname SET Posts = $Posts WHERE Usernam
+e = '$Username'");
  $sth->execute() or die "Cannot execute: " . $sth->errstr();
  }

$sth = $dbh->prepare("SELECT Username, Email, Posts, Timestamp FROM no
+name");
$sth->execute() or die "Cannot execute: " . $sth->errstr();

while ((my $Username, my $Email, my $Posts, my $Timestamp) = $sth->fet
+chrow_array ())
  {
  next if ! defined $Timestamp;
  print "$Username, $Email, $Posts, $Timestamp\n";
  }

$sth->finish();
[download]

Prints:

Username,  Email,  Posts,  Timestamp
ed, ed@there, 11, 10:23:01
bill, bill@there, 5, 10:23:02
Username,  Email, 1,  Timestamp
ed, ed@there, 12, 10:23:01
bill, bill@there, 6, 10:23:02
[download]