It may just be that \$test is not a reference to an int. In any case, I'd be rewriting add_nir(). I think the following does what you want: 
use warnings;
use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

void add_nir(int x, int y, SV * ref_int) {
     sv_setiv(ref_int, x + y);
}

EOC

my $test;
add_nir(1, 4, $test);
print $test, "\n"; # prints "5"

[download]
I also seem to recall that there can be problems when one Inline function tries to access an Inline'd function in a module - but I'm not sure of the details. It's not all that difficult to remove the Inline dependency from a module (thereby making it the same as any other XS module). <plug>InlineX::C2XS and InlineX::CPP2XS can help with that </plug>.

Cheers,
Rob

In reply to Re^5: Inline CPP by syphilis
in thread Inline CPP by nirf1

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