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.