Man, that's one ugly regex. And I say that as a guy who's written a lot of ugly regexes. :)

I _think_ (kinda hard to tell from your misformatted "desired answer" line) you're looking for something like this:

#!/usr/bin/perl
use common::sense;

my $data = do { local $/; <DATA>; };
$data =~ s/\n(?!\[)/ /gs;
for (split /\n/, $data){
    my @line = split /[\[\] ]+|ERRORMSG /, $_, 6;
    print join(" === ", @line[1..3,5]), "\n";
}

__DATA__
[2012/02/16 00:08:34] [29] ERRORMSG unknown error Can't insert into pr
+ice table
Please check
Valueprice.pm line 52.
[2012/02/16 00:08:34] [39] ERRORMSG Invalid User
[2012/02/16 00:14:52] [105] ERRORMSG missing conversion rate
[2012/02/16 00:14:52] [29] ERRORMSG Can't use an undefined value as a 
+HASH reference at Value.pm line 77.
[download]

Prints:

2012/02/16 === 00:08:34 === 29 === unknown error Can't insert into pri
+ce table Please check Valueprice.pm line 52.
2012/02/16 === 00:08:34 === 39 === Invalid User
2012/02/16 === 00:14:52 === 105 === missing conversion rate
2012/02/16 === 00:14:52 === 29 === Can't use an undefined value as a H
+ASH reference at Value.pm line 77.
[download]

Is that what you're looking for?

Update: Whoops - I think I just figured out what the OP is asking... so there are two problems in his code. Revised solution.

-- 
I hate storms, but calms undermine my spirits.
 -- Bernard Moitessier, "The Long Way"