From perlrun:

NOTE: Use of -P is strongly discouraged because of its inherent problems, including poor portability. It is deprecated and will be removed in a future version of Perl.

...

If you’re considering using "-P", you might also want to look at the Filter::cpp module from CPAN.

Update: I'm too slow. Oh well. Why do you want the preprocessor, anyways? I'd think that using a plain old variable would work just fine in that circumstance.