use warnings;
use strict;

my ($to_be_printed);

while (<DATA>) {
    
    chomp;
    
    if (/\d\d-\w{3}-\d\d/) {
        
        print $to_be_printed, "\n" if $to_be_printed;
        
        $to_be_printed = $_;
        
        next;
        
    }
    
    $to_be_printed .= " $_";
    
}

print $to_be_printed, "\n";

__DATA__
23-Dec-07 12:34 A B C
D E F G
H I JJ KK L
23-Dec-07 12:35 A B C D E
F GG HH
II J K L M
N O P Q
24-Dec-08 15:15 AA B C D
E F GG