my $sep = '##';
my $sep_len = length($sep);

my $last_pos = index($str, $sep);

for (;;) {
  my $pos = index($str, '##', $last_pos + $sep_len);

  my $rec_pos = $last_pos;

  if ($pos < 0) {
     last if $last_pos == length($str);
     $last_pos = length($str);
  } else {
     $last_pos = $pos;
  }

  my $rid = substr($str, $rec_pos+19, 10);
  print "$rid: " . ($keeplist{$rid} ? 'y' : 'n') . "\n";

}