Well, if you don't like any of the suggestions that have been made so far, you might try taking the source POSIX.xs from the source 5.00402 and recompile it. This may involve recompiling other modules to resolve dependencies. But still, technically this will be a "correct" way.

Look, everyone here wants to help you, but you have a hopeless situation. Perhaps you should try a little harder to convince the management to upgrade Perl on the server to a more modern version (if binary compatibility is a problem, 5.6.1 should be good), or write a script that doesn't use POSIX. For the snippet shown in POSIX.pm and Perl 5.004_02 - Compatible?, this should be easy.