#!/usr/bin/perl -w
use strict;

my %ERROR;
my %ERROR2;
my @classes = ('H','S','O');
my $output = <<EOD;
4865FA9B   0702 P H rmt0           TAPE OPERATION ERROR
DE9A52D1   0704 I S rmt0           DEVICE DUMP RETRIEVED
4865FA9B   0701 P H rmt2           TAPE OPERATION ERROR
F3E9B3E2   0620 I O SYSJ2          UNABLE TO ALLOCATE SPACE IN FILE SYSTEM
DCB47997   0511 T H hdisk4         DISK OPERATION ERROR
EOD

# populate original hash
for my $line (split /\n/,$output) {
   chomp $line;
   my ($IDENTIFIER,$TIMESTAMP,$T,$C,$RES,$DESC) = split(/\s+/,$line,6);
   $ERROR{$C}{$RES}{$DESC}{time} = $TIMESTAMP;
   $ERROR{$C}{$RES}{$DESC}{id} = $IDENTIFIER;
   $ERROR{$C}{$RES}{$DESC}{T} = $T;
}
# elsewhere, I create a secondary hash for sorting.
my $counter = 0;

for my $CLASS (@classes) {
   for my $RES (keys %{$ERROR{$CLASS}}) {
      for my $DESC (keys %{$ERROR{$CLASS}{$RES}}) {
         $counter++;
         $ERROR2{$CLASS}{$counter}{time} = $ERROR{$CLASS}{$RES}{$DESC}{time};
         $ERROR2{$CLASS}{$counter}{desc} = $DESC;
         # ... transfer some other data from %ERROR to %ERROR2
      }
   }
}
# ...
# output - SORT EACH CLASS BY MOST RECENT TIME. 
# sort using CLASS as primary and time as secondary key.
# note how RES and DESC have no influence on the sorting.

for my $CLASS (@classes) {
  for my $nr (reverse sort {$ERROR2{$CLASS}{$a}{time} <=> $ERROR2{$CLASS}{$b}{time} }
                keys %{$ERROR2{$CLASS}}) {
      print "CLASS $CLASS : Time : $ERROR2{$CLASS}{$nr}{time} Description: $ERROR2{$CLASS}{$nr}{desc}\n";
   }
}