I suppose the lib to be loaded needs to be a shared (aka dynamic) object — from your .o extension I figure it is not.

Compile your C test code as follows (and then load the .so file):

$ cc -c -fPIC test.c
$ cc -shared test.o -o test.so
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Update: just tried it, and although it works fine (when compiling as shown above) with code samples such as

int test()      // ->DeclareSub("test", "i")
{
   return 42;
}

void test()     // ->DeclareSub("test", "")
{
    printf("WORKS!\n");
}
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I couldn't get it to work with your sample returning a char* (I get an "Illegal instruction" exception (SIGILL)).  Apparently, "p" is not the appropriate type specifier here...