Something like this should get you headed in the right direction:

use strict;
use warnings;

get_avr ('cluster1');

sub get_avr {
    my $cluster = shift;
    my %stats;
    
    while (<DATA>) {
        my ($date, $hour, $minutes, $load) = /(\S+)\s+(\d+)(\S+)\s+(\S
++)/;
    
        $stats{$hour}->[0] += $load;
        $stats{$hour}->[1]++;
    }
    
    for (sort keys %stats) {
        print "$cluster $_:00 " . $stats{$_}[0] / $stats{$_}[1] . "\n"
+;
    }
}

__DATA__
05/08/06 00:00 51.84 1915510 1061 66.13
05/08/06 01:00 25.67 1255510 648 66.07
05/08/06 00:00 35.56 2077540 739 66.26
05/08/06 01:00 33.91 2262510 718 66.37
[download]

Prints:

cluster1 00:00 43.7
cluster1 01:00 29.79
[download]