It's not made any easier by what gets done behind the scenes:

use warnings;

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

use Inline C => <<'EOC';

void foo1 (int x) {
     Inline_Stack_Vars;
     Inline_Stack_Reset;
     Inline_Stack_Push(sv_2mortal(newSVuv(x * 3)));
     Inline_Stack_Done;
     Inline_Stack_Return(1);
}

SV * foo2 (int x) {
     return newSVuv(x * 3);
}

EOC

print foo1(7), "\n", foo2(8), "\n";
[download]

Notice that foo1() specifically mortalises, while foo2() does not. Seems strange - but if you look at the C file that gets generated you'll see that the mortalisation got inserted into foo2() for you.

Although I've used an Inline::C script as a demo, it's xsubpp behaviour - nothing to do with Inline at all.

Cheers,
Rob