Sometimes I forget how insane Perl was before `use strict;` became commonplace.
use strict; has basically eliminated typos or variable-related errors for me. combined with -T, it's given me a lot of peace of mind, which I miss when using PHP. I'm seriously contemplating writing my own SetConst and GetConst functions. (Lately I've been finding a lot of sense in immutable variables.)
use const name => value;
Though ... I'm not sure why you'd need a "set" constant, as constants should be immutable, and set once ...