The -P command-line switch was deprecated in perl version 5.10.0 and removed as of version 5.12.0 (see perl5120delta). But the equivalent behaviour is provided by the module Filter::cpp:

#! perl
use strict;
use warnings;
use Filter::cpp;

#define DEBUG
#define XYZ 42

#ifdef DEBUG
print "Debugging enabled\n";
#else
print "Debugging disabled\n";
#endif

print XYZ . "\n";
[download]

Output:

13:14 >perl 772_SoPW.pl
Debugging enabled
42

13:14 >
[download]

See perlfilter.

You're right, of course. For some reason I had a terminal window open with a Perl 5.10.1 environment and chose that one to run perldoc instead of anything more recent.