my $mutex = $mutexes{$k} ||= do { my $mutex : shared; \$mutex };

##</code><code>##

my $mutex = $mutexes{$k};
if (! defined $mutex) {
   lock($mutexes);
    if (!defined $mutexes{$k}) {
          my $new_mutex : shared; 
          $mutexes{$k} = \$new_mutex 
          $mutex = \$new_mutex
    }
}