ThirtySecondNoob:

You're not that far off with your attempt. You're going through a few unnecessary steps to check whether the file is open and for storing the file handle, though. I've cleaned up your example a little, here (usual disclaimers apply):

my $prev_file_name;
my $file;
while(1)
{
   my $new_line = get_line_of_code();
   my $line_time = get_date_stamp($new_line);

   my $file_name = "$DATA_DIRECTORY$line_time.ascii";
   if (defined($prev_file_name) and $prev_file_name != $file_name) {
      close($file) if defined $file;
      open $file, '>>', $file_name
        or print "$file_name could not be created\n";
   }

   #add line to $NEW_FILE
   print $file $new_line."\n"
     or print "data COULDNT be printed to CURRENT_FILE\n";

   # This bit seems to be related to code you've pruned from
   # your example...
   #} else {
   #   #write to file and send
   #   $file = $fileholder[0];
   #   print $file $new_line;
   #}
} #end of while loop
[download]

While going through your example, though, I feel it's possible you may have trimmed a bit too much from it. In case you wanted to keep several different files open at once (such as for processing interleaved requests), you might want something like this:

my $MAX_FILES=10; # Max open files we want
my %files;        # We can keep multiple file handles
while(1)
{
   my $new_line = get_line_of_code();
   my $line_time = get_date_stamp($new_line);

   my $file_name = "$DATA_DIRECTORY$line_time.ascii";
   if (!exists $files{$file_name}) {
      # close a file if we have too many
      my @filenames = keys %files;
      if (@filenames >= $MAX_FILES) {
         my $t = $files{$filenames[0]};
         close $t or die $!;
         delete $files{$filenames[0]};
      }

      open $files{$file_name}, '>>', $filename
        or print "$file_name could not be created\n";
   }

   $file = $files{$file_name};
   print $file $new_line."\n"
     or print "data COULDNT be printed to CURRENT_FILE\n";
} #end of while loop
[download]

...roboticus