Strictures (strict and warnings) turn on various extra checking for dubious practices or programming errors. use strict requires (among many other things) that variables are declared (using my) and that various techniques that often cause confusion or have been made redundant in newer versions of Perl are not used.

use warnings makes various run time checks. The most important one is that variables have been initialized before their contents are used. Together these two strictures provide an extremely useful early warning system for many common coding mistakes. Always use strictures in Perl. ;)