embedded perl in C, perlapio - interoperability with STDIO

I just realized, that the PerlIO layer seems to do something more than just (more or less) easily wrap the stdio.h-functions. If I try to use a file-descriptor resolved via PerlIO_stdout() and PerlIO_fileno() with functions from stdio.h, this fails.

For example:

PerlIO* perlStdErr = PerlIO_stderr();
fdStdErrOriginal = PerlIO_fileno(perlStdErr);
relocatedStdErr = dup(fdStdOutOriginal);
_write(relocatedStdErr, "something", 8); //<-- this fails

I've tried this with VC10. The embedded perl program is executed from a different context - so it's not possible to use PerlIO from the context where the write to the relocatedStdErr is performed.

For the curious: I need to execute a perl script and forward the output of the script's stdout/stderr to a log whilst keeping the ability to write on stdout for myself. Moreover this should work platform independent (linux, windows console application, win32 desktop application). Just to forward the stdout/stderr doesn't work in Win32 desktop applications since there is none ;) - you need to use the perl's stdout/stderr.

Needed solution: Be able to write on a filehandle (or descriptor) derived from perlio NOT using the perlio stack.