May be I'm misunderstanding you, but is this what you want?:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];

my %categs;
push @{ $categs{ $_->[2] } }, $_->[1] while @{ $_ = [ split ' ', <DATA
+> ] };

pp \%categs;

__DATA__
31 John Consultation Charges 100 4/1/2012 10:39
805 Kennedy Consultation Charges 150 4/1/2012 11:17
805 Kennedy C-Reactive Protein 170 4/1/2012 11:56
805 Kennedy Complete Blood Count 150 4/1/2012 11:56
805 Kennedy Malarial 175 4/1/2012 11:56
805 Kennedy Mantoux Test 100 4/1/2012 11:56
805 Kennedy AZIBACT 1 MG SYP 28 4/1/2012 13:27
805 Kennedy FALCINILLE DRY SYP 105.15 4/1/2012 13:27
891 Ruth Consultation Charges 150 4/1/2012 12:05
891 Ruth C-Reactive Protein 170 4/1/2012 12:47
891 Ruth Complete Blood Count 150 4/1/2012 12:47
891 Ruth Mantoux Test 100 4/1/2012 12:47
891 Ruth X-Ray Chest 150 4/1/2012 12:47
[download]

Producing:

C:\test>junk59
{
  AZIBACT      => ["Kennedy"],
  "C-Reactive" => ["Kennedy", "Ruth"],
  Complete     => ["Kennedy", "Ruth"],
  Consultation => ["John", "Kennedy", "Ruth"],
  FALCINILLE   => ["Kennedy"],
  Malarial     => ["Kennedy"],
  Mantoux      => ["Kennedy", "Ruth"],
  "X-Ray"      => ["Ruth"],
}
[download]