diff --git a/perl/Changes b/perl/Changes
index dc3dd5c..5d5a5e2 100644
--- a/perl/Changes
+++ b/perl/Changes
@@ -1,3 +1,8 @@
+0.21
+
+    - doc enhancment
+    - micro performance tuning.
+
 0.20
 
     - first production ready release with PP driver.
diff --git a/perl/README b/perl/README
index d5fc269..dec6125 100644
--- a/perl/README
+++ b/perl/README
@@ -32,8 +32,14 @@ ABOUT MESSAGEPACK FORMAT
     visit to <http://msgpack.org/>.
 
 METHODS
-    my $packed = Data::MessagePack->pack($data);
-        pack the $data to messagepack format string.
+    my $packed = Data::MessagePack->pack($data[, $max_depth]);
+        Pack the $data to messagepack format string.
+
+        This method throws exception when nesting perl structure more than
+        $max_depth(default: 512) for detecting circular reference.
+
+        Data::MessagePack->pack() throws exception when encountered blessed
+        object. Because MessagePack is language independent format.
 
     my $unpacked = Data::MessagePack->unpack($msgpackstr);
         unpack the $msgpackstr to messagepack format string.
diff --git a/perl/lib/Data/MessagePack.pm b/perl/lib/Data/MessagePack.pm
index 0229bca..197d744 100644
--- a/perl/lib/Data/MessagePack.pm
+++ b/perl/lib/Data/MessagePack.pm
@@ -3,7 +3,7 @@ use strict;
 use warnings;
 use 5.008001;
 
-our $VERSION = '0.20';
+our $VERSION = '0.21';
 our $PreferInteger = 0;
 
 our $true  = do { bless \(my $dummy = 1), "Data::MessagePack::Boolean" };