Use the force Luke!

/* hash structure: */
/* This structure must match the beginning of struct xpvmg in sv.h. */
struct xpvhv {
    HV*         xmg_stash;      /* class package */
    union _xmgu xmg_u;
    STRLEN      xhv_keys;       /* total keys, including placeholders 
+*/
    STRLEN      xhv_max;        /* subscript of last element of xhv_ar
+ray */
};
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and

/* the number of keys (including any placeholders) */
#define XHvTOTALKEYS(xhv)       ((xhv)->xhv_keys)

/*
 * HvKEYS gets the number of keys that actually exist(), and is provid
+ed
 * for backwards compatibility with old XS code. The core uses HvUSEDK
+EYS
 * (keys, excluding placeholders) and HvTOTALKEYS (including placehold
+ers)
 */
#define HvKEYS(hv)              HvUSEDKEYS(hv)
#define HvUSEDKEYS(hv)          (HvTOTALKEYS(hv) - HvPLACEHOLDERS_get(
+hv))
#define HvTOTALKEYS(hv)         XHvTOTALKEYS((XPVHV*)  SvANY(hv))
#define HvPLACEHOLDERS(hv)      (*Perl_hv_placeholders_p(aTHX_ MUTABLE
+_HV(hv)))
#define HvPLACEHOLDERS_get(hv)  (SvMAGIC(hv) ? Perl_hv_placeholders_ge
+t(aTHX_ (const HV *)hv) : 0)
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HvKEYS(hv) is the thing that implements scalar keys %hash