my %file_cache;
sub open_file($) {
  my $filename = shift;
  return $file_cache{$filename} ||
         (open $file_cache{$filename}, ">>$filename" or
            die "blah blah $!");
}


while (<FILE> {
  foreach my $n (@natures) {
    if ($_ =~ /%n/) {
      my $out = open_file "$n.txt";
      print $out $n;
    }
  }
}