#!/usr/bin/perl --
use Path::Class;
use constant THISFILE => "".file( __FILE__ )->absolute;
use constant THISDIR  => "".file( THISFILE )->dir;
BEGIN { chdir THISDIR or die $!; }
use Inline (
    C                 =>
    Config            =>
    LIBS              => "-Llib/relative -lrelative",
    INC               => "-Iinc/relative",
    BUILD_NOISY       => 1,
    CLEAN_AFTER_BUILD => 0,
    DIRECTORY         => 'RELATIVE', # INVALID VALUE?! IT MUST EXIST
);
use Inline C => file('a.c')->absolute, qw' NAME RelativeA ';
use Inline C => file('b.c')->absolute, qw' NAME RelativeB ';

use strict; use warnings;
...