The main problem with source filters is that they must parse Perl code in order to transform it, and Perl code is very hard to parse. Perl has a lot of context sensitivity and parsing it involves more that a straight lexing-parsing data flow; there is a subtle interaction between the lexing and the parsing that even has probabilistic aspects to it.

This is what folks mean when they say that only perl can parse Perl :) Thus source filters are playing with code they do not fully understand, and could be considered dangerous.

If you wnat to use Switch.pm in production code, I'd recommend coding stanards to keep the conditional for the cases reasonbly simple, and test always.