The lock doesn't have to the be on the variable you are changing, so create a hash of mutexes. 

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;
}

[download]

Update: Fixed error mentioned in reply.

In reply to Re: Avoid Locking Entire Hashes by ikegami
in thread Avoid Locking Entire Hashes by jagan_1234

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