Writing a log file with threads

Here are all the threads write the log file. I want to know if the below script is fine, if it needs any modifications, and whether I am utilising threads properly to execute this code fast.

open(LOG,"+>","log.txt")or die "cant open the file";


my $file_total= scalar @listOfFiles;

    my $div = $file_total/$thread_count;
    $div = ceil($div);
    my $start = 0;
    my $end = $div;

    for ($count = 1; $count <=$thread_count ; $count++) 
    {
        my $thread = threads->new(\&register, $start, $end );
        push(@threads,$thread);        
        $start = $end;
        $end = $end + $div;
        if ($end > $file_total)
        {
            $end = $file_total;
        }
    }

    foreach (@threads) 
    {
       $_->join;
    }
    close(LOG);
    sub register
    {
         my $lstart = shift;
         my $lend = shift;

         for (my $index = $lstart; $index < $lend; ++$index) 
         {
             print LOG $listOfFiles[$index];
             process......
             ..........
             .......
         }
     }

process placeholder: I have a huge number of documents, say, 1 million. I am collecting all the 1 million filenames, storing them in the listOfFiles array, and dividing the array index based on the number of threads the user wants. I am passing every thread and the start and end indices to fetch the document name, process the document, and store the status of the output to a log file.