diff --git a/perl/MessagePack.c b/perl/MessagePack.c
index 3b33fee1..f56d1476 100644
--- a/perl/MessagePack.c
+++ b/perl/MessagePack.c
@@ -19,10 +19,14 @@ XS(xs_unpacker_is_finished);
 XS(xs_unpacker_data);
 XS(xs_unpacker_reset);
 
+void boot_Data__MessagePack_pack(void);
+
 XS(boot_Data__MessagePack) {
     dXSARGS;
     HV * stash;
 
+    boot_Data__MessagePack_pack();
+
     newXS("Data::MessagePack::pack", xs_pack, __FILE__);
     newXS("Data::MessagePack::unpack", xs_unpack, __FILE__);
     stash = gv_stashpvn("Data::MessagePack", strlen("Data::MessagePack"), TRUE);
diff --git a/perl/lib/Data/MessagePack.pm b/perl/lib/Data/MessagePack.pm
index 712d3b5c..ff179e8a 100644
--- a/perl/lib/Data/MessagePack.pm
+++ b/perl/lib/Data/MessagePack.pm
@@ -5,6 +5,7 @@ use XSLoader;
 use 5.008001;
 
 our $VERSION = '0.02';
+our $PreferInteger = 0;
 
 XSLoader::load(__PACKAGE__, $VERSION);
 
diff --git a/perl/pack.c b/perl/pack.c
index c8987fef..03e65303 100644
--- a/perl/pack.c
+++ b/perl/pack.c
@@ -51,6 +51,38 @@ void need(enc_t *enc, STRLEN len)
     }
 }
 
+
+static int s_pref_int = 0;
+
+static int pref_int_set(pTHX_ SV* sv, MAGIC* mg) {
+    if (SvTRUE(sv)) {
+        s_pref_int = 1;
+    } else {
+        s_pref_int = 0;
+    }
+    return 0;
+}
+
+MGVTBL pref_int_vtbl = {
+    NULL,
+    pref_int_set,
+    NULL,
+    NULL,
+    NULL,
+    NULL,
+    NULL,
+#ifdef MGf_LOCAL
+    NULL,
+#endif
+};
+
+void boot_Data__MessagePack_pack(void) {
+    SV* var = get_sv("Data::MessagePack::PreferInteger", 0);
+    sv_magicext(var, NULL, PERL_MAGIC_ext, &pref_int_vtbl, NULL, 0);
+    SvSETMAGIC(var);
+}
+
+
 static int looks_like_int(const char *str, size_t len) {
     int i;
     for (i=0; i<len; i++) {
@@ -133,8 +165,7 @@ static void _msgpack_pack_sv(enc_t *enc, SV* val) {
             char * cval = SvPV(val, len);
             const int U32_STRLEN = 10; /* length(0xFFFFFFFF) */
 
-            SV* pref_int = get_sv("Data::MessagePack::PreferInteger", 0);
-            if (pref_int && SvTRUE(pref_int) && len <= U32_STRLEN && looks_like_int(cval, len) && SvUV(val) < U32_MAX) {
+            if (s_pref_int && len <= U32_STRLEN && looks_like_int(cval, len) && SvUV(val) < U32_MAX) {
                 PACK_WRAPPER(uint32)(enc, SvUV(val));
                 return;
             }