diff --git a/perl/MessagePack.c b/perl/MessagePack.c
index e5835268..6b9010cd 100644
--- a/perl/MessagePack.c
+++ b/perl/MessagePack.c
@@ -20,10 +20,14 @@ XS(xs_unpacker_data);
 XS(xs_unpacker_reset);
 XS(xs_unpacker_destroy);
 
+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 2368a5b8..e2046694 100644
--- a/perl/lib/Data/MessagePack.pm
+++ b/perl/lib/Data/MessagePack.pm
@@ -5,6 +5,7 @@ use XSLoader;
 use 5.008001;
 
 our $VERSION = '0.03';
+our $PreferInteger = 0;
 
 XSLoader::load(__PACKAGE__, $VERSION);
 
diff --git a/perl/pack.c b/perl/pack.c
index c8987fef..b329551b 100644
--- a/perl/pack.c
+++ b/perl/pack.c
@@ -26,7 +26,7 @@ typedef struct {
     char *end; /* SvEND (sv) */
     SV *sv;    /* result scalar */
 } enc_t;
-void need(enc_t *enc, STRLEN len);
+static void need(enc_t *enc, STRLEN len);
 
 #define msgpack_pack_user enc_t*
 
@@ -41,7 +41,7 @@ void need(enc_t *enc, STRLEN len);
 #define PACK_WRAPPER(t) _PACK_WRAPPER(t)
 #define INIT_SIZE   32 /* initial scalar size to be allocated */
 
-void need(enc_t *enc, STRLEN len)
+static void need(enc_t *enc, STRLEN len)
 {
     if (enc->cur + len >= enc->end) {
         STRLEN cur = enc->cur - (char *)SvPVX (enc->sv);
@@ -51,16 +51,92 @@ void need(enc_t *enc, STRLEN len)
     }
 }
 
-static int looks_like_int(const char *str, size_t len) {
-    int i;
-    for (i=0; i<len; i++) {
-        if (!isDIGIT(str[i])) {
-            return 0;
-        }
+
+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 try_int(enc_t* enc, const char *p, size_t len) {
+    int negative = 0;
+    const char* pe = p + len;
+    uint64_t num = 0;
+
+    if (len == 0) { return 0; }
+
+    if (*p == '-') {
+        /* length(-0x80000000) == 11 */
+        if (len <= 1 || len > 11) { return 0; }
+        negative = 1;
+        ++p;
+    } else {
+        /* length(0xFFFFFFFF) == 10 */
+        if (len > 10) { return 0; }
+    }
+
+#if '9'=='8'+1 && '8'=='7'+1 && '7'=='6'+1 && '6'=='5'+1 && '5'=='4'+1 \
+               && '4'=='3'+1 && '3'=='2'+1 && '2'=='1'+1 && '1'=='0'+1
+    do {
+        unsigned int c = ((int)*(p++)) - '0';
+        if (c > 9) { return 0; }
+        num = num * 10 + c;
+    } while(p < pe);
+#else
+    do {
+        switch (*(p++)) {
+        case '0': num = num * 10 + 0; break;
+        case '1': num = num * 10 + 1; break;
+        case '2': num = num * 10 + 2; break;
+        case '3': num = num * 10 + 3; break;
+        case '4': num = num * 10 + 4; break;
+        case '5': num = num * 10 + 5; break;
+        case '6': num = num * 10 + 6; break;
+        case '7': num = num * 10 + 7; break;
+        case '8': num = num * 10 + 8; break;
+        case '9': num = num * 10 + 9; break;
+        default: return 0;
+        }
+    } while(p < pe);
+#endif
+
+    if (negative) {
+        if (num > 0x80000000) { return 0; }
+        msgpack_pack_int32(enc, ((int32_t)num) * -1);
+    } else {
+        if (num > 0xFFFFFFFF) { return 0; }
+        msgpack_pack_uint32(enc, (uint32_t)num);
+    }
+
     return 1;
 }
 
+
 static void _msgpack_pack_sv(enc_t *enc, SV* val) {
     if (val==NULL) {
         msgpack_pack_nil(enc);
@@ -131,11 +207,8 @@ static void _msgpack_pack_sv(enc_t *enc, SV* val) {
         if (SvPOKp(val)) {
             STRLEN len;
             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) {
-                PACK_WRAPPER(uint32)(enc, SvUV(val));
+            if (s_pref_int && try_int(enc, cval, len)) {
                 return;
             }
 
diff --git a/perl/t/05_preferred_int.t b/perl/t/05_preferred_int.t
index 176430eb..39c725a6 100644
--- a/perl/t/05_preferred_int.t
+++ b/perl/t/05_preferred_int.t
@@ -17,15 +17,24 @@ sub pis ($$) {
 }
 
 my @dat = (
+    '',      'a0',
     '0',     '00',
     '1',     '01',
-    '10',     '0a',
+    '10',    '0a',
+    '-1',    'ff',
+    '-10',   'f6',
+    '-',     'a1 2d',
     ''.0xEFFF => 'cd ef ff',
     ''.0xFFFF => 'cd ff ff',
     ''.0xFFFFFF => 'ce 00 ff ff ff',
-    ''.0xFFFFFFFF => 'aa 34 32 39 34 39 36 37 32 39 35',
+    ''.0xFFFFFFFF => 'ce ff ff ff ff',
     ''.0xFFFFFFFFF => 'ab 36 38 37 31 39 34 37 36 37 33 35',
     ''.0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFF => 'b4 38 2e 33 30 37 36 37 34 39 37 33 36 35 35 37 32 65 2b 33 34',
+    '-'.0x8000000 => 'd2 f8 00 00 00',
+    '-'.0x80000000 => 'd2 80 00 00 00',
+    '-'.0x800000000 => 'ac 2d 33 34 33 35 39 37 33 38 33 36 38',
+    '-'.0x8000000000 => 'ad 2d 35 34 39 37 35 35 38 31 33 38 38 38',
+    '-'.0x800000000000000000000000000000 => 'b5 2d 36 2e 36 34 36 31 33 39 39 37 38 39 32 34 35 38 65 2b 33 35',
     {'0' => '1'}, '81 00 01',
     {'abc' => '1'}, '81 a3 61 62 63 01',
 );