use strict;
use warnings;
use List::Util qw(sum);

my %data;
while (<DATA>) {
    my ($time, $value) = (split)[2..3];
    next unless $time =~ /:/;
    my $hour = (split /:/, $time)[0];
    push @{ $data{$hour} }, $value;
}

for my $hour (sort keys %data) {
    my @vals = @{ $data{$hour} };
    print "Hour=$hour, Count=", scalar @vals, " Avg=", sum(@vals)/@val
+s, "\n";
}

__DATA__
itemid  FROM_UNIXTIME(clock)       value
26603   2010-10-09 00:00:23     38784360.8136
26603   2010-10-09 00:01:23     36529742.6667
26603   2010-10-09 00:02:23     36966880.8000
26603   2010-10-09 00:03:23     35666405.8667
26603   2010-10-09 01:05:23     35600190.6667
26603   2010-10-09 02:06:23     39175029.2000
26603   2010-10-09 03:07:23     35386478.4000
26603   2010-10-09 04:08:27     35773482.7500
26603   2010-10-09 05:09:23     39073367.5714
26603   2010-10-09 06:10:23     34379049.8667
26603   2010-10-09 07:11:24     33984045.7705
26603   2010-10-09 09:12:23     36276301.8305
26603   2010-10-09 10:13:23     37248366.0000
up to time 23:59
26603   2010-10-09 23:59:23     39393421.3333
[download]

prints:

Hour=00, Count=4 Avg=36986847.53675
Hour=01, Count=1 Avg=35600190.6667
Hour=02, Count=1 Avg=39175029.2
Hour=03, Count=1 Avg=35386478.4
Hour=04, Count=1 Avg=35773482.75
Hour=05, Count=1 Avg=39073367.5714
Hour=06, Count=1 Avg=34379049.8667
Hour=07, Count=1 Avg=33984045.7705
Hour=09, Count=1 Avg=36276301.8305
Hour=10, Count=1 Avg=37248366
Hour=23, Count=1 Avg=39393421.3333
[download]