Thanks a lot CombatSquirrel, that was very close to what I'm looking for. I did
rewrite the date part, as I have decided to use unix timestamp, as that simplifies things some.
And may I say you RoCk! here's, the updated code:

#!/usr/bin/perl
#
use warnings;
use strict;
use Data::Dumper;
sub apache2epoch()
{
use POSIX;
my $datetime=$ARGV[0];
my $i=0;
my %months;
%months = map { $_ => $i++ } ("Jan","Feb","Mar","Apr","May","Jun",
                              "Jul","Aug","Sep","Oct","Nov","Dec");
$_[0] =~ m((\d+)/(\w+)/(\d+):(\d+):(\d+):(\d+));
return  &mktime($6,$5,$4,$1,$months{$2},$3-1900,0,0,-1);
}                                                                     
+                                                                     
+      
my %users;                           
while (<DATA>) {
my ($user,      $time,  $primary,
    $secondary, $views, $clicks,
    $sessions,  $mode,  $zip) =  split;
if ($users{$user}) {
if ((&apache2epoch($time))-(&apache2epoch(($users{$user}->{LastVisitTi
+me}))) >1800 ) {
         $users{$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__
188c0530ac92475a 01/Jan/2003:16:00:02 Computers Computers 1 1 1 no_mod
+e 44614
0ce46e475f94ecb9 01/Jan/2003:16:00:08 Computers Computers 1 0 1 no_mod
+e 00000
189e45678964fcf6 01/Jan/2003:16:00:07 Electronics Electronics 1 0 1 no
+_mode 00000
189a4a75d0cbad03 01/Jan/2003:16:00:01 No_category No_category 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
0ce46e475f94ecb9 01/Jan/2003:17:00:08 Computers Computers 1 0 1 no_mod
+e 00000
189a4a75d0cbad03 01/Jan/2003:17:00:01 No_category No_category 1 0 1 no
+_mode 00
[download]

btw, on the CompDates sub, if I encounter the same $user it gets the errors
Use of uninitialized value in numeric comparison (<=>) at ./overall.pl line 28, <DATA> line 7.
thanks again CombatSquirrel, and Cheers to you too

~vili
sniff sniff 802.11