In real (production) code, you should also verify that you in fact have the appropriate types (using SvROK(), SvIOK(), etc.) before treating them as such

Yes, SvROK(*aref) should be checked, but checking SvIOK(*res) is not required because SvIV coerces.

#!/usr/bin/perl

use Inline C => <<'EOC';

void show(SV* index) {
    PerlIO_printf(PerlIO_stdout(), "%d\n", SvIOK(index));
    PerlIO_printf(PerlIO_stdout(), "%d\n", SvIV(index));
}

EOC

show("123456");
[download]

0
123456
[download]