use warnings;
use strict;
use Data::Dumper;

my %data;

while(<DATA>){
    my ($server_name,$fault) = (split/:|\s+/,$_)[0,2];
    $data{$server_name}{$fault}++;
}

print Dumper \%data;

__DATA__
Server1:london:network_interface
Server1:london:diskspace
Server1:london:diskpace
Server1:london:kernel
Server2:paris:diskspace
Server3:new_york:Kernel
Server3:new_york:diskspace
Server3:new_york:diskspace
Server3:new_york:kernel

##</code><code>##

$VAR1 = {
          'Server3' => {
                         'kernel' => 1,
                         'diskspace' => 2,
                         'Kernel' => 1
                       },
          'Server1' => {
                         'kernel' => 1,
                         'diskpace' => 1,
                         'diskspace' => 1,
                         'network_interface' => 1
                       },
          'Server2' => {
                         'diskspace' => 1
                       }
        };