It looks a little repetitive. I think this is pretty much the same:

  # Somewhere at the top
  my $chunk = qr/[A-Z0-9()-]{2,15})\s+([A-Z0-9()-]{7}/;\
  #...
  # then instead of your loop body
  if (/^(\d{1,5})\s+($chunk)/g) {
      my $1st = $1;
      my $subrecord_id = 1;
      ++$record_count;
      while (/\G($chunk)/g) {
          print OUTFILE "$record_count\t$1st\t$subrecord_id\t$_\n";
          $subrecord_id++;
      }
  }
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(unless you might have lines that start with the pattern, but then do something different).