Might any of the XS-based modules that your script uses been compiled with a compiler other than VC++ 6? Or perhaps an external library that a module relies on? This in principle can lead to problems. For example, using modules compiled with VC++ 7 with ActivePerl (which is compiled with VC++ 6) will load two different C runtime libraries (msvcrt.dll and msvcrt7x.dll). While this might not be a problem most of the time, there can be instances where a conflict arises - for example, using one of the dlls to allocate some memory and then later using the other to try to free it.