Warning, Carp was rewritten from scratch in Perl 5.6 and Perl 5.8. It could easily be rewritten again in Perl 5.10. If so, then undocumented internal functions like ret_backtrace() would be unlikely to be maintained.

I strongly recommend that you write your own version of that (which isn't hard using caller). Furthermore the advice to not go liberally borrowing undocumented internals is basic good programming advice that applies to any modules that you aren't maintaining.