my $finished = 0 : shared; 
sub write_to_file
{
  until ( $finished )
  {
    open ( my $temp_fh, ">>", $temp_file );
    print $temp_fh "aabbcc\n";
    close ( $temp_fh );
  }
}


#main bit of code here...

$finished = 1;
foreach my $thread ( threads -> list() )
{
  $thread -> join();
}