You can get and set the current length of the string stored in an SV
    with the following macros:

        SvCUR(SV*)
        SvCUR_set(SV*, I32 val)

    You can also get a pointer to the end of the string stored in the SV
    with the macro:

        SvEND(SV*)

    But note that these last three macros are valid only if "SvPOK()" is
    true.