#!/usr/bin/perl

use strict;

my %record;
my $previous_event;
my $max_event = 4;
my $key = 1;

while (my $line = <DATA>) {
    chomp( $line );
    my ($current_event) = $line =~ /EVENT(\d)/;
    $record{$key}->{$current_event} = $line;

    $key++ if ( $current_event == 4 && 
      $previous_event != $current_event );
    $previous_event = $current_event;
}

for my $i (sort keys %record) {
    print map { $record{$i}->{$_} . "\n" } 
      sort keys %{ $record{$i} };
}

__DATA__
09:12:50:861 EVENT1
09:13:09:467 EVENT1
09:13:09:837 EVENT1
09:13:38:059 EVENT2
09:14:03:115 EVENT3
09:14:04:076 EVENT4
09:14:11:376 EVENT1
09:14:34:049 EVENT2
09:14:34:990 EVENT3
09:14:34:990 EVENT3
09:14:34:990 EVENT4