Actually, PL_hints is just used at compile-time, not at runtime. But using (PL_curcop->op_private & HINT_STRICT_STRINGIFY) instead should work.

UPDATE: Oh, and a minor note: Perl_warner needs an additional argument that indicates the warning category, like: 

if ((PL_curcop->op_private & HINT_STRICT_STRINGIFY)
                && ckWARN(WARN_MISC))
    Perl_warner(aTHX_ packWARN(WARN_MISC),
        "Stringification of reference disallowed");

[download]
or alternatively you could omit the check and use Perl_warn which takes no such argument.

In reply to Re^3: How to prevent references from stringifying? by xmath
in thread How to prevent references from stringifying? by diotalevi

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