How to set a Perl environment variable from an XSUB?

I am trying to set a Perl environment variable from an XSUB. I want it to take immediate effect before the XSUB exits. Here is my XS file, Module.xs:

#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

MODULE = My::Module  PACKAGE = My::Module
PROTOTYPES: DISABLE

void
set_env_test()
    CODE:
        I32 croak_on_error = 1;
        eval_pv("$ENV{PERL_MEM_LOG}='ms'", croak_on_error);
        printf("C1: getenv : %s\n", getenv("PERL_MEM_LOG"));
        printf("C1: PerlEnv_getenv : %s\n", PerlEnv_getenv("PERL_MEM_LOG"));
        my_setenv("PERL_MEM_LOG", "s");
        printf("C2: getenv : %s\n", getenv("PERL_MEM_LOG"));
        printf("C2: PerlEnv_getenv : %s\n", PerlEnv_getenv("PERL_MEM_LOG"));

and here is a Perl script that uses the XSUB:

use feature qw(say);
use strict;
use warnings;
use ExtUtils::testlib;
use My::Module;

{
    say "Before: ", get_env();
    My::Module::set_env_test();
    say "After: ", get_env();
}

sub get_env {
    if (exists $ENV{PERL_MEM_LOG}) {
        return $ENV{PERL_MEM_LOG};
    }
    else {
        return "undef";
    }
}

The output is:

Before: undef
C1: getenv : ms
C1: PerlEnv_getenv : ms
C2: getenv : s
C2: PerlEnv_getenv : s
After: ms

I would like to know if it is possible to set the environment variable without using eval_pv()? Is there a specific API function that I can use?

Observations:

eval_pv() works as expected,
my_setenv() works locally and supersedes the value set by eval_pv() until the XSUB exits, then the old value is restored.
my_setenv() cannot be used to set the Perl environment variable permanently, its effect only lasts within the XSUB.
getenv() and PerlEnv_getenv() seems to do the same thing (?)

Solution

I don't know why my_setenv doesn't work (since $ENV{PERL_MEM_LOG} = "abc"; ends up calling my_setenv), but the following does:

HV *env_hv = get_hv("ENV", 0);
if (!env_hv)
   croak("wut");

SV **svp = hv_fetchs(env_hv, "PERL_MEM_LOG", 1);
sv_setpvs_mg(*svp, "s");

Test:

use 5.014;
use warnings;

use Inline C => <<'__EOS__';

    void set_env_test() {
        I32 croak_on_error = 1;
        eval_pv("$ENV{PERL_MEM_LOG}='ms'", croak_on_error);
        printf("C1: getenv : %s\n", getenv("PERL_MEM_LOG"));
        printf("C1: PerlEnv_getenv : %s\n", PerlEnv_getenv("PERL_MEM_LOG"));

        HV *env_hv = get_hv("ENV", 0);
        if (!env_hv)
           croak("wut");

        SV **svp = hv_fetchs(env_hv, "PERL_MEM_LOG", 1);
        sv_setpvs_mg(*svp, "s");

        printf("C2: getenv : %s\n", getenv("PERL_MEM_LOG"));
        printf("C2: PerlEnv_getenv : %s\n", PerlEnv_getenv("PERL_MEM_LOG"));
    }

__EOS__

sub get_env { $ENV{PERL_MEM_LOG} // "[undef]" }

{
    say "Before: ", get_env();
    set_env_test();
    say "After: ", get_env();
}