what about:

while(<>) {
 chomp;
   if(/^FT.*/) {
     $ft_flag ++;
     next;
    }
   if(/^FH.*/) {
     $fh_flag ++;
     next;
    }
 if($ft_flag) {...; next;}
 if($fh_flag) {...; next;}
 if(/^BH.*/) {
    $current_bh=$_;
    next;
  }
  if(^0006.*) {
    do_whatever_using($current_bh,$_);
    next;
   }
}
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