Looking at this if one assumes that each record starts with ]a[, and that you want to group all the records by the value in this key field, you can collect the records as listed below. This caputures one or more record on a line, though the solution assumes that each record is never spans multiple lines. I also assume that the markers for each field is always a single character except for 'a' (case insensitive).

-albert

#!/usr/bin/perl -w

my $key;
my $records;

while(<DATA>){
    ($key) =  /\][Aa]\[(.*?)(?=[\]\n])/ if /\][Aa]\[/;
    my @items = /\]([b-zB-Z0-9])\[(.*?)(?=[\]\n])/g if /\][b-zB-Z0-9]\
+[/;
    foreach (my $i = 0; $i < @items; $i += 2){
        $records->{$key}->{$items[$i]} = $items[$i+1];
    }
}

foreach my $k(sort keys %$records){
    foreach my $item (sort keys %{$records->{$k}}){
        # Do stuff with found records here
        print join("\t", $k, $item, $records->{$k}->{$item}), "\n";
    }
}

__DATA__
]a[1
]b[FORTUNE BAY]1[R74]c[(9)]d[58
]e[b g 9 Little Brown Jug-Gemfly (by Golden Elder (GB))]f[(BLINKERS)
]g[A W & Mrs P I Reynolds, T A Robinson, D J & Mrs V G Stuart & A R Wa
+ghorn
]h[Gay Stuart, Riverton
]i[Emerald green, navy blue braces & diamond armbands, emerald green c
+ap
]j[04640]w[CANT 13 Aug H Rating 76 1600m 11 of 16 Tailend inr,btld clo
+ser inr T C Newton 58 (6) 15\
/15 Wnr: BRIGHTON PIER (AUS) 1:42.69 13.8L
]k[ :(46-2-7-6-6-$21145)
]m[ F(0-0-0) G(17-1-5) D(7-0-1) S(15-1-6) H(7-0-1) C(1-0-0) Dst(2-0-0)

]a[2
]b[LUCKY LAD]1[R72]c[(10)]d[57
]e[ch g 6 Shinko King (IRE)-Argyll Dazzler (by Tarrago (ITY))]f[(BLINK
+ERS)
]g[D H F Green
]h[Kevin Hughes, Riccarton Park
]i[Emerald green & light blue diagonal stripes, striped sleeves & cap
]j[X3333]w[CANT 10 Aug H Rating 76 1800m 3 of 15 Trld ldr,2nd tn,fght 
+mid N G Harris 57 (4) 1/1 Wn\
r: SHRKAN 2:01.69 4.5L
]k[ :(28-1-4-7-2-$12688)
]m[ F(0-0-0) G(13-0-4) D(7-0-3) S(6-1-3) H(2-0-1) C(0-0-0) Dst(3-0-2)
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