my %h : shared;
my %mutexes : shared;

sub get_mutex {
   my ($k) = @_;

   my $mutex_ref = $mutexes{$k};
   return $mutex_ref if $mutex_ref;

   lock($mutexes);
   my $new_mutex : shared;
   return $mutexes{$k} ||= \$new_mutex;
}

sub safe_set {
   my ($k, $v) = @_;
   lock ${ get_mutex($k) };
   $h{$k} = $v;
}