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

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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.

In reply to Re^3: Intercept any changes to the sv_refcnt/SvREFCNT of an object by Corion
in thread Intercept any changes to the sv_refcnt/SvREFCNT of an object by vernonlyon

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