I haven't coded in C in many years, and I've never used Inline::C ever, but I took a stab. When I simplify your C code, then change __END__ to __DATA__ in the module, I can get the C to compile, and things *appear* to work correctly. eg:

Module:

package c_calc;
use Inline C;
1;
__DATA__
__C__
int test(int num){
    return num;
}
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Script:

use warnings;
use strict;

use lib '.';
use c_calc;

my $num = c_calc::test(5);

print $num;
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Might be worth a try. Let us know if it works or not, as inevitably, one or more of the Inline experts will be around eventually.