Perl_av_extend(pTHX_AV *av, I32 key)
{
MAGIC * const mg = SvTIED_mg((SV*)av, PERL_MAGIC_tied);
if (msg) {
...
PUSHs(SvTIED_obj((SV*)av, mg));
PUSHs(sv_2mortal(newSViv(key+1)));
PUTBACK;
call_method("EXTEND", G_SCALAR|G_DISCARD);