Actually, the Inline module makes it possible to have C code that is compiled without changing your Makefile.PL on a number of systems. Now, whether the C code itself is portable depends on what it calls (for instance, there is no portable way to determine the number of processors). But it can be possible to make portable C routines that compile across platforms. Here's an example from the Inline::C::Cookbook :

use Inline C => <<'END_C';
    void greet() {
        printf("Hello, world\n");
    }
END_C

greet;
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