If you follow the Very Good Advice of keeping most of your non-display code in external libraries and you run under mod_perl, make sure you fully stop&start apache whenever you make a change, to ensure you see the results.

Consider developing in CGI mode (no mod_perl,) with a single apache process, with code_cache_max_size => 0 (disabled, no caching) to avoid some such weirdness and speed development.