Maybe a simple approach?

use strict;
use warnings;

my @log = ();
while (<DATA>){
    chomp;
    push @log, $_;
    if (scalar@log ==3){
        do_stuff(\@log);
        @log = ();
    }
}

sub do_stuff{
    my $ref = shift;
    print join ":", @$ref, "\n";
}
__DATA__
1
2
3
4
5
6
[download]

Gives :

1:2:3:
4:5:6:
[download]

Argh. Only just seen your update... what solution did you go for in the end?

:) yours :P

Sorry for commenting after you already found a solution. I too am curious as to how you solved it.

I like BioLion's solution, except I would add regular expressions to definitively identify which of the 3 log lines you have. In other words, make sure that you are grouping the correct 3 lines together.

Since this is a log file, and log files are commonly rotated and/or truncated, you cannot be sure that the first line in your log file is the first line in a 3-line group. It is safer to not make that assumption.

Well, there is more work to do here, as I have run into the problem where I am not alwyas grabbing the correct 3 lines. I will post a solution once I get it complete...I have a few ideas, I will just have to delve into my perl books to see how to implement that bit I think...hmmmmmmm