diff --git a/perl/lib/Data/MessagePack/PP.pm b/perl/lib/Data/MessagePack/PP.pm
index 8a904a97..44940dec 100644
--- a/perl/lib/Data/MessagePack/PP.pm
+++ b/perl/lib/Data/MessagePack/PP.pm
@@ -22,10 +22,19 @@ BEGIN {
     if(!eval { pack 'Q', 1 }) { # don't have quad types
         $unpack_int64_slow = sub {
             require Math::BigInt;
-            my $high = Math::BigInt->new( unpack_int32(  $_[0], $_[1]) );
-            my $low  = Math::BigInt->new( unpack_uint32( $_[0], $_[1] + 4) );
+            my $high = unpack_uint32( $_[0], $_[1] );
+            my $low  = unpack_uint32( $_[0], $_[1] + 4);
 
-            return +($high << 32 | $low)->bstr;
+            if($high < 0xF0000000) { # positive
+                $high = Math::BigInt->new( $high );
+                $low  = Math::BigInt->new( $low  );
+                return +($high << 32 | $low)->bstr;
+            }
+            else { # negative
+                $high = Math::BigInt->new( ~$high );
+                $low  = Math::BigInt->new( ~$low  );
+                return +( -($high << 32 | $low + 1) )->bstr;
+            }
         };
         $unpack_uint64_slow = sub {
             require Math::BigInt;
diff --git a/perl/t/data.pl b/perl/t/data.pl
index 8ffd25ae..95eac419 100644
--- a/perl/t/data.pl
+++ b/perl/t/data.pl
@@ -35,4 +35,9 @@ no warnings; # i need this, i need this.
 
     'd3 ff ff ff ff ff ff ff ff' => '-1',                    # int64_t
     'cf ff ff ff ff ff ff ff ff' => '18446744073709551615',  # uint64_t
+
+    # int64_t
+    'd3 00 00 00 10 00 00 00 00' => '68719476736',
+    'd3 00 00 00 10 00 00 00 01' => '68719476737',
+    'd3 10 00 00 00 00 00 00 00' => '1152921504606846976',
 )