For me, $data->{API} doesn't exist ... not sure of the best way to fix this. Turns out the reason I couldn't generate the error is that I now don't run using the '-w' switch. Instead I 'use warnings;' .... which doesn't set $^W. (This in itself is an Inline::C bug.)

If I've understood, I can fairly easily re-create your situation. If I create a Foo.pm that looks like:

package Foo;

use Inline C => <<'END_OF_C_CODE';

double * foo() {
  double a = 123.456, *x;
  x = &a;
  return x;
}

END_OF_C_CODE

1;
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and a 'test.pl' that looks like:

#!perl -w
use Foo;

use Inline C => <<'END_OF_C_CODE';

int foo2() {
    int x = 1234567890;
    return x;
}

END_OF_C_CODE

print "All compiled\n";
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then when I run 'perl test.pl' I get:

D:\pscrpt\inline>perl test.pl
Warning. No Inline C functions bound to Perl
Check your C function definition(s) for Inline compatibility

All compiled
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with no indication that the warning was in reference to Foo.pm only.

Is it ok with you if I file reports at rt.cpan about both bugs ? ... or would you prefer to attend to it yourself ?.
Probably leave it up to Ingy as how he wants to deal with them. He is (or claims to be :-) currently working towards the release of Inline-0.45.

Cheers,
Rob