It does/can contain:

/* flags (note that type modifiers can also be used as flags!) */
#define FLAG_WAS_UTF8          0x40
#define FLAG_PARSE_UTF8       0x20    /* Parse as utf8 */
#define FLAG_UNPACK_ONLY_ONE  0x10
#define FLAG_DO_UTF8          0x08    /* The underlying string is utf8
+ */
#define FLAG_SLASH            0x04
#define FLAG_COMMA            0x02
#define FLAG_PACK             0x01
[download]

See Perl_unpackstring in pp_Pack.c

At a cursory glance, the only useful input value seems to be FLAG_DO_UTF8, all the others are set internally, and even that seems to be checked and set if need internally.

Ie. set to 0 and let it work it out until you need to know more.