Source filters alter the program text of a module before Perl sees it, much
 as a C preprocessor alters the source text of a C program before the
 compiler sees it.
 .
 This package contains a number of source filters, including:
 .
   exec and sh: pipe the source file through an external command
   cpp: pipe the source file through the C preprocessor
   decrypt: example of simple (though weak!) source obfuscation
   tee: copy filtered source to a file (debugging aid)
 .
 A substantial amount of documentation and examples is also included.