main.d:

    import core.stdc.stdio;
    import core.stdc.stdlib;
    import core.sys.posix.dlfcn;

    extern (C) int dll();

    int main()
    {
        printf("+main()\n");

        void *lh = dlopen("libdll.so", RTLD_LAZY);
        if (!lh)
        {
            fprintf(stderr, "dlopen error: %s\n", dlerror());
            exit(1);
        }
        printf("libdll.so is loaded\n");

        int function() fn = cast(int function())dlsym(lh, "dll");
        char *error = dlerror();
        if (error)
        {
            fprintf(stderr, "dlsym error: %s\n", error);
            exit(1);
        }
        printf("dll() function is found\n");

        fn();

        printf("unloading libdll.so\n");
        dlclose(lh);

        printf("-main()\n");
        return 0;
    }

dll.d:

    import core.stdc.stdio;

    extern (C) int dll()
    {
        printf("dll()\n");
        return 0;
    }

    static this()
    {
        printf("libdll.so construction\n");
    }

    static ~this()
    {
        printf("libdll.so destruction\n");
    }

Build:

    dmd -c dll.d -fPIC
    dmd -oflibdll.so dll.o -shared -defaultlib=libphobos2.so -L-rpath=/home/walter/cbx/mars/phobos/generated/linux/release/64

    dmd -c main.d
    dmd main.o -L-ldl -defaultlib=libphobos2.so -L-rpath=.:/home/walter/cbx/mars/phobos/generated/linux/release/64
    ./main

Results:

    +main()
    libdll.so is loaded
    dll() function is found
    dll()
    unloading libdll.so
    -main()

Note that the module constructors and destructors are not run. This is a bug.