Rather than a big block of if then elsifs, you might consider using a dispatch table:

#! perl -slw
use strict;

sub other{ print "Processing '$_[0]' with arg: '$_[1]'" }

my %dispatch = map{ $_ => \&other } 'b'..'z';
$dispatch{a} = sub {
    print "\nNew section\nProcessing 'a' with arg '$_[1]'";
};

while( <DATA> ) {
    chomp;
    $dispatch{$1}->( $1, $2 )
        while m/ \] ( [a-z] ) \[ ( [^]]+ ) /xg;
}

__DATA__
...
[download]

P:\test>489332

New section
Processing 'a' with arg '1'
Processing 'b' with arg: 'FORTUNE BAY'
Processing 'c' with arg: '(9)'
Processing 'd' with arg: '58'
Processing 'e' with arg: 'b g 9 Little Brown Jug-Gemfly (by Golden Eld
+er (GB))'
Processing 'f' with arg: '(BLINKERS)'
Processing 'g' with arg: 'A W & Mrs P I Reynolds, T A Robinson, D J & 
+Mrs V G Stuart & A R Waghorn'
Processing 'h' with arg: 'Gay Stuart, Riverton'
Processing 'i' with arg: 'Emerald green, navy blue braces & diamond ar
+mbands, emerald green cap'
Processing 'j' with arg: '04640'
Processing 'w' with arg: 'CANT 13 Aug H Rating 76 1600m 11 of 16 Taile
+nd inr,btld closer inr TC Newton 58 (6) 15/15 Wnr: BRIGHTON PIER (AUS
+) 1:42.69 13.8L
Processing 'k' with arg: ' :(46-2-7-6-6-$21145)'
Processing 'm' with arg: ' 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)'

New section
Processing 'a' with arg '2'
Processing 'b' with arg: 'LUCKY LAD'
Processing 'c' with arg: '(10)'
Processing 'd' with arg: '57'
Processing 'e' with arg: 'ch g 6 Shinko King (IRE)-Argyll Dazzler (by 
+Tarrago (ITY))'
Processing 'f' with arg: '(BLINKERS)'
Processing 'g' with arg: 'D H F Green'
Processing 'h' with arg: 'Kevin Hughes, Riccarton Park'
Processing 'i' with arg: 'Emerald green & light blue diagonal stripes,
+ striped sleeves & cap'
Processing 'j' with arg: 'X3333'
Processing 'w' with arg: 'CANT 10 Aug H Rating 76 1800m 3 of 15 Trld l
+dr,2nd tn,fght mid N G Harris 57 (4) 1/1 Wnr: SHRKAN 2:01.69 4.5L'
Processing 'k' with arg: ' :(28-1-4-7-2-$12688)'
Processing 'm' with arg: ' 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)'

New section
Processing 'a' with arg '3'
[download]