#!/usr/bin/perl --

use strict;
use warnings;

my $line = "";
OUTER:
while (<DATA>) {
    if (/^\[/) { # could use \[\d{4}
        $line = $_;
    }
  INNER:
    my $i = 0;
    while (<DATA>) {
        if (/^\[/) {
            print $line, "\n" if $i;
            $line = $_;
            next OUTER;
        }
        $line .= $_;
        $i++;
    }
}

__DATA__
[2008/10/14 10:57:18] [55] DEBUG Austin::API::vendor_api - Initialisin
+g the API Galileo
[2008/10/14 10:57:18] [55] DEBUG Austin::API::vendor_api - Initialisin
+g the API Galileo
[2008/10/14 10:57:18] [55] DEBUG Austin::API::vendor_api - Initialisin
+g the API Galileo
[2008/10/14 10:57:18] [41] DEBUG Austin::Suppliers::Common::Location::
+init - Setting Up Generic Locations
[2008/10/14 11:15:22] [147] DEBUG Austin::Controller::default - $VAR1 
+= {
          'Errors' => [
                        {
                          'ID' => 'noRates',
                          'Error' => 'No Rates for given dates/times',
                          'object' => '/var/website/modules/Austin/CB/
+Search.pm',
                          'Location' => undef,
                          'line' => 371
                        },
                      ]
        };
[2008/10/14 10:57:18] [245] DEBUG Austin::XV::Trip::set_time_zone - Ch
+ecking timezone with agent UK
[2008/10/14 10:57:18] [247] DEBUG Austin::XV::Trip::set_time_zone - Se
+tting timezone to Europe/London
[2008/10/14 11:15:22]
[2008/10/14 11:15:22] [147] DEBUG Austin::Controller::default - $VAR1 
+= {
          'Errors' => [
                        {
                          'ID' => 'noRates',
                          'Error' => 'No Rates for given dates/times',
                          'object' => '/var/website/modules/Austin/CB/
+Search.pm',
                          'Location' => undef,
                          'line' => 371
                        },
                      ]
        };
[2008/10/14 11:15:22]
[download]

Also worth investigating is sliding window technique, ex Matching in huge files