A simple approach.

use strict;
use warnings;

my $found = '';

while(<DATA>){

  chomp;

  if($_=~ /^\.\.AU:$/) { $found = 'AU';   next; }
  if($found eq 'AU'){    print "\n".$_; }

  if($_=~ /^\.\.BD:$/) { $found = 'BD';   next; }
  if($found eq 'BD'){    print ','.$_;  }

  $found = '';
}
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UPDATE: Could have been shorter ;)

my $tag;

while(<DATA>){

  chomp;

  if(/([A-Z]{2}):$/){ $tag = $1; next; }

  if($tag eq 'AU'){ print "\n".$_; }
  if($tag eq 'BD'){ print  ','.$_; }
}
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