Taking a closer look, I think there are also other problems with your code. In particular, you're not (Perl-)dereferencing (SvRV()) the arrayref you get from hv_fetch() as the value associated with the hash entry "permutation". Also, you're not checking that the SV** pointer which hv_fetch() returns is non-null before (C-)dereferencing it...

Somewhat simplified, the sequence of steps should look something like this (I'm using Inline::C here for ease of demo):

#!/usr/bin/perl -l

use Inline C => <<'EOC';

int get(SV* self, int index) {

    HV* h     = (HV*) SvRV(self);
    char* key = "permutation";
    SV** aref = hv_fetch(h, key, strlen(key), FALSE);
    if (aref == NULL) { /* ... */  exit(-1); }
    AV* a     = (AV*) SvRV(*aref);
    SV** res  = av_fetch(a, index, FALSE);
    if (res  == NULL) { /* ... */  exit(-1); }
    IV i      = SvIV(*res);

    return i;
}

EOC

my $obj = bless { permutation => [2,42,2,2,2,2,2,2,2,2,2] }, "Whatever
+";

print get($obj, 1);   # prints 42
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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.

(As for chromatic's note below, the mapping of the Perl integer to the C int index is being taken care of by Inline::C in this example.)

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");
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0
123456
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thanks, through your suggestion I was able to fix my code :)