while (@data = <DATA>) {
    ...
}
[download]

doesn't make sense. The expression @data = <DATA> reads all lines from the filehandle DATA into @data. End while. Drop it.

Instad of

for($i=0;$i<$#data;$i++){
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better say

for my $line (@data) {
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That said, I'd do

my @tokens;
while (<DATA>) {
    if (/\.\.(AU|BD):/) {
        if ($1 eq 'AU') {
            print join( ',',@tokens),"\n", @tokens = () if @tokens;
        }
        chomp (my $line = <DATA>);
        push @tokens, $line;        
    }
}
print join( ',',@tokens),"\n", @tokens = () if @tokens;
[download]