As incrementing/decrementing the reference count happens (very) often, these are implemented as C macros and not hookable function calls in sv.c:

#if defined(__GNUC__) && !defined(PERL_GCC_BRACE_GROUPS_FORBIDDEN)
#  define SvREFCNT_inc(sv)        \
    ({                    \
    SV * const _sv = MUTABLE_SV(sv);    \
    if (_sv)            \
         (SvREFCNT(_sv))++;        \
    _sv;                \
    })
#  define SvREFCNT_inc_simple(sv)    \
    ({                    \
    if (sv)                \
         (SvREFCNT(sv))++;        \
    MUTABLE_SV(sv);                \
    })
#  define SvREFCNT_inc_NN(sv)        \
    ({                    \
    SV * const _sv = MUTABLE_SV(sv);    \
    SvREFCNT(_sv)++;        \
    _sv;                \
    })
#  define SvREFCNT_inc_void(sv)        \
    ({                    \
    SV * const _sv = MUTABLE_SV(sv);    \
    if (_sv)            \
        (void)(SvREFCNT(_sv)++);    \
    })
#else
#  define SvREFCNT_inc(sv)    \
    ((PL_Sv=MUTABLE_SV(sv)) ? (++(SvREFCNT(PL_Sv)),PL_Sv) : NULL)
#  define SvREFCNT_inc_simple(sv) \
    ((sv) ? (SvREFCNT(sv)++,MUTABLE_SV(sv)) : NULL)
#  define SvREFCNT_inc_NN(sv) \
    (PL_Sv=MUTABLE_SV(sv),++(SvREFCNT(PL_Sv)),PL_Sv)
#  define SvREFCNT_inc_void(sv) \
    (void)((PL_Sv=MUTABLE_SV(sv)) ? ++(SvREFCNT(PL_Sv)) : 0)
#endif
[download]

The "easy" way is to just redefine these macros and recompile Perl. You should be aware that invoking Perl code from within such a hook would be very unwise, as Perl code inevitably will allocate values whose refcount needs to be incremented, which would invoke your hook, recursively.

This is also the hard way.