Checking if your kit is complete...
Looks good
Writing Makefile for Devel::WeakRef

Microsoft (R) Program Maintenance Utility   Version 6.00.8168.0
Copyright (C) Microsoft Corp 1988-1998. All rights reserved.

cp lib/Devel/WeakRef.pm blib\lib\Devel\WeakRef.pm
	C:\cpanrun\build\5-10-0\bin\perl.exe C:\cpanrun\build\5-10-0\lib\ExtUtils\xsubpp  -typemap C:\cpanrun\build\5-10-0\lib\ExtUtils\typemap  WeakRef.xs > WeakRef.xsc && C:\cpanrun\build\5-10-0\bin\perl.exe -MExtUtils::Command -e mv WeakRef.xsc WeakRef.c
Please specify prototyping behavior for WeakRef.xs (see perlxs manual)
	cl -c    -nologo -GF -W3 -MD -Zi -DNDEBUG -O1 -DWIN32 -D_CONSOLE -DNO_STRICT -DHAVE_DES_FCRYPT -DUSE_SITECUSTOMIZE -DPRIVLIB_LAST_IN_INC -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO -DPERL_MSVCRT_READFIX -MD -Zi -DNDEBUG -O1    -DVERSION=\"0.003\"  -DXS_VERSION=\"0.003\"  "-IC:\cpanrun\build\5-10-0\lib\CORE"   WeakRef.c
WeakRef.c
WeakRef.xs(42) : warning C4113: 'int (__cdecl *)(struct sv *,struct magic *)' differs in parameter lists from 'int (__cdecl *)(struct interpreter *,struct sv *,struct magic *)'
WeakRef.xs(54) : warning C4113: 'int (__cdecl *)(struct sv *,struct magic *)' differs in parameter lists from 'int (__cdecl *)(struct interpreter *,struct sv *,struct magic *)'
WeakRef.xs(54) : warning C4113: 'int (__cdecl *)(struct sv *,struct magic *)' differs in parameter lists from 'int (__cdecl *)(struct interpreter *,struct sv *,struct magic *)'
WeakRef.xs(119) : error C2065: 'sv_undef' : undeclared identifier
WeakRef.xs(119) : warning C4133: 'function' : incompatible types - from 'int *' to 'struct sv *'
NMAKE : fatal error U1077: 'cl' : return code '0x2'
Stop.