I agree with browserUK on this one, but would add also:

- use 'use strict;' in your sources, to ensure no auto-declarations are allowed.

- avoid global variables where possible and use 'use constant;' for the cases where you want a global constant like pi to be immutable and ubiquitously available.

- use classes (and possibly Moo) to reduce the need for randomly scoped identifiers and to manage the scope and access to attributes.