How about the following code:

#!perl

use strict;
use warnings;

use Data::Dumper;

my %monthLookup = (
       Jan =>  1,
       Feb =>  2,
       Mar =>  3,
       Apr =>  4,
       May =>  5,
       Jun =>  6,
       Jul =>  7,
       Aug =>  8,
       Sep =>  9,
       Oct => 10,
       Nov => 11,
       Dec => 12  );

sub CompDates($$) {
   my ($first, $second) = @_;
   $first  = [($first  =~ m!(\d{2])/(\w{3})/(\d{4}):(\d{2}):(\d{2}):(\
+d{2})!)];
   $second = [($second =~ m!(\d{2])/(\w{3})/(\d{4}):(\d{2}):(\d{2}):(\
+d{2})!)];
   $first->[1]  = $monthLookup{$first->[1]};
   $second->[1] = $monthLookup{$second->[1]};
   return (($first->[0] <=> $second->[0]) ||
           ($first->[1] <=> $second->[1]) ||
           ($first->[2] <=> $second->[2]) ||
           ($first->[3] <=> $second->[3]) ||
           ($first->[4] <=> $second->[4]) ||
           ($first->[5] <=> $second->[5])  );
}

my %users;

while (<DATA>) {
   my ($user,      $time,  $primary,
       $secondary, $views, $clicks,
       $sessions,  $mode,  $zip) =  split;
   if ($users{$user}) {
      if (CompDates($users{$user}->{LastVisitTime}, $time) < 0) {
         $users{$user}->{LastVisitTime} = $time;
      }
   } else {
      $users{$user}->{LastVisitTime}     = $time;
      $users{$user}->{PrimaryCategory}   = $primary;
      $users{$user}->{SecondaryCategory} = $secondary;
      $users{$user}->{MarketingMode}     = $mode;
      $users{$user}->{ZIP}               = $zip;
   }
   $users{$user}->{PageViews}      += $views;
   $users{$user}->{MerchantClicks} += $clicks;
   $users{$user}->{Sessions}       += $sessions;
}

print Dumper(\%users);

__DATA__
0ce46e475f94ecb9 01/Jan/2003:16:00:08 Computers Computers 1 0 1 no_mod
+e 00000
188c0530ac92475a 01/Jan/2003:16:00:02 Computers Computers 1 1 1 no_mod
+e 44614
189a4a75d0cbad03 01/Jan/2003:16:00:01 No_category No_category 1 0 1 no
+_mode 00000
189e45678964fcf6 01/Jan/2003:16:00:07 Electronics Electronics 1 0 1 no
+_mode 00000
18a416ba3d3c7a8d 01/Jan/2003:16:00:12 No_category No_category 2 0 1 no
+_mode 00000
18aa11982e30e1ef 01/Jan/2003:16:00:07 No_category No_category 1 0 1 no
+_mode 00000
[download]

This should do it, as far as I understood your problem. Post a reply if it does not.
Cheers, CombatSquirrel.