It seems to me the "uninitialized value" warning is undef getting turned into 0

Yes, I think that's right:

use warnings;

use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

void foo(int a) {}

void bar(SV * a) {}

void baz(SV * a) {
     int i = (int)SvIV(a);
}
    
EOC

$u = undef;

foo($u); # warning
bar($u); # no warning
baz($u); # warning
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Cheers,
Rob