Here are 2 additional solutions. One uses DateTime and the other uses Date::Parse and POSIX. I don't know about speed, but you could try them out and see. Also, which is the most easy to read might be a consideration, (as you stated).

#!/usr/bin/perl
use strict;
use warnings;
use DateTime::Format::Strptime;

my $dt = DateTime::Format::Strptime->new( pattern => '%Y.%m.%d %H:%M')
+;

chomp(my @line = split /,/, <DATA>);
my @data = \@line;

my $beg = $dt->parse_datetime("@line[0,1]")->truncate( to => 'hour' );
my $end = $beg->clone->add(hours => 12);

printf "%s  -+-  %s\n", map tr/T/ /r, $beg, $end;

while (<DATA>) {
    chomp(my @line = split /,/);
    my $date = $dt->parse_datetime("@line[0,1]")->truncate( to => 'hou
+r' );
    
    if ($date < $end) {
        push @data, \@line;    
    }
    else {
        process(@data);
        
        @data = \@line;
        $end = $date->clone->add(hours => 12);
        
        printf "%s  -+-  %s\n", map tr/T/ /r, $date, $end;
    }
}

process(@data);

sub process {
    my @data = @_;
    # do somthing with data
    my @sum;
    my @idx = 2 .. 5;
    for my $line (@data) {
        for my $col (@idx) {
            $sum[$col] += $line->[$col];
        }
    }
    printf "Avg of %d lines", scalar @data;
    print +(map {sprintf "%10.5f", $sum[$_] / @data} @idx), "\n";
}
[download]

The Date::Parse solution is nearly the same.

#!/usr/bin/perl
use strict;
use warnings;
use Date::Parse qw/ str2time /;
use POSIX qw/ strftime /;

chomp(my @line = split /,/, <DATA>);
my @data = \@line;

my $beg = str2time("@line[0,1]");
$beg = int($beg / 3600) * 3600; # truncate minutes and seconds

my $end = $beg + 12 * 60 * 60; # add 12 hours

printf "%s  -+-  %s\n",
    map {strftime "%Y-%m-%d %H:%M:%S", localtime $_} $beg,$end;

while (<DATA>) {
    chomp(my @line = split /,/);
    my $date = str2time("@line[0,1]");
    $date = int($date / 3600) * 3600; # truncate minutes and seconds
    
    if ($date < $end) {
        push @data, \@line;    
    }
    else {
        process(@data);
        
        @data = \@line;
        $end = $date + 12 * 60 * 60;
        
        printf "%s  -+-  %s\n",
            map {strftime "%Y-%m-%d %H:%M:%S", localtime $_} $date, $e
+nd;
    }
}

process(@data);

sub process {
    my @data = @_;
    # do somthing with data
    my @sum;
    my @idx = 2 .. 5;
    for my $line (@data) {
        for my $col (@idx) {
            $sum[$col] += $line->[$col];
        }
    }
    printf "Avg of %d lines", scalar @data;
    print +(map {sprintf "%10.5f", $sum[$_] / @data} @idx), "\n";
}

__DATA__
2011.01.13,21:25,1.33508,1.33524,1.33470,1.33494,391
2011.01.13,21:30,1.33494,1.33506,1.33447,1.33453,318
2011.01.13,21:35,1.33453,1.33483,1.33417,1.33434,426
2011.01.13,21:40,1.33434,1.33468,1.33417,1.33467,309
2011.01.13,21:45,1.33471,1.33493,1.33465,1.33465,233
2011.01.13,21:50,1.33465,1.33475,1.33443,1.33463,184
2011.01.13,21:55,1.33463,1.33519,1.33463,1.33493,344
2011.01.13,22:00,1.33494,1.33563,1.33489,1.33524,318
2011.01.13,22:05,1.33524,1.33551,1.33512,1.33549,182
2011.01.14,22:05,1.33524,1.33551,1.33512,1.33549,182
[download]

These produced the output:

C:\Old_Data\perlp>perl t33.pl
2011-01-13 21:00:00  -+-  2011-01-14 09:00:00
Avg of 9 lines   1.33478   1.33509   1.33458   1.33482
2011-01-14 22:00:00  -+-  2011-01-15 10:00:00
Avg of 1 lines   1.33524   1.33551   1.33512   1.33549
[download]

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