in reply to C pre-processor in Perl

Because it doesn't work very well. In particular, the C preprocessor doesn't know Perl syntax. Take this tiny example and save it into

#define FOO "bar" print qq(The value of FOO is "), FOO, qq("\n);
and then try it out:
$ perl The value of FOO is "FOO" $ perl -P The value of "bar" is "FOO" $
It expands it when it shouldn't and doesn't when it should.

I expect that you could get many C preprocessors to choke or issue warnings on fairly common Perl syntax as well.

This is enough of a reason for me to not use such a trick. There are several other good reasons to not use this given in the thread. One of which I ran into just trying to demonstrate how my tiny example fails. First, on Win32 even with MS VC++ installed:

> perl -P 'cl' is not recognized as an internal or external command, operable program or batch file. > vcvars32 Setting environment for using Microsoft Visual C++ tools. > perl -P Command line error D2003 : missing source filename >
Random Unix box:
$ perl -P ngcc: -: No such file or directory ngcc: No input files specified. $

                - tye