The -P flag turns loose the C preprocessor on your source before perl ever sees it. Its behavior is determined by the C standard, at least for the portable parts. The handiest source of information is probably a C primer.

In short, the C preprocessor makes direct textual substitution. with some logic available. I've never needed it for perl, but potentially useful constructs include,

• #include "filename" - inclusion of source from another file.
• #define foo some string or other - substitution macro definition
• #if, #elseif, #else, and friends allow for conditional inclusion of source.

None of this is really necessary to perl.

After Compline,
Zaxo