Using file system based locking and re-opening the file for every message is ...urm... lets just say, non-optimal.

The simplest, most portable way if you are using threads is to use a Thread::Queue.

use threads;
use Thread::Queue;
...
my $Q = new Thread::Queue;

sub logmsg {
  // added thread id to messages.
  $Q->enqueue( threads->tid . ": @_"; );
}

async {
    open my $log, '>', 'logfile' or die $!;
    while( $Q->dequeue ) {
        my $now = time;
        my $msec = int( ( $now - int($now) ) * 1000 );
        my ( $sec, $min, $hour ) = localtime(time);

        sprintf $log "%02d:%02d:%02d:%03d %s\r\n", $hour, $min, $sec, 
+$msec, $_; 
    }
    close $log;
}

... 

logmsg( ... );

...

$Q->enqueue( undef ); // to terminate logging thread.
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• There is no need for file locking as all the writing is performed by a single thread.

  The queue takes care of itself.

• Loggin causes minimal delays in the critical code path.

  Queuing a message is very fast as all the formatting and IO is done in another thread.

• The ordering of messages is preserved.

  Queues are ordered.

Simple, effective and very efficient.