As a starter:

use warnings;

use Inline C => Config =>
    CLEAN_AFTER_BUILD => 0,
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

int testSub(SV * x) {
    if(!SvROK(x)) croak ("Not a reference");
    sv_setpv(SvRV(x), "Hello!;)\n\0");
    return 1;
}

EOC

$init = "barbarella";
$test_var = \$init;

testSub($test_var);

print "$$test_var\n$init\n";
[download]

For me, that outputs:

Hello!;)

Hello!;)
[download]

If you specifically need to see the XS file that Inline::C autogenerated for this particular exercise, you'll find it in the build directory (./_Inline/build).

Cheers,
Rob