Perl

Robust scripts

All scripts should start with the following:

use strict;
use warnings;
use utf8;
use autodie;

Only remove one or more of the above if you really know what you are doing.

It is also a good idea to define a minimum Perl version, e.g. to require Perl 5.14 or above:

use v5.14;

More readable names for common variables (e.g. $/ can be referred to as $INPUT_RECORD_SEPARATOR) can be created with:

use English;

Although used interchangeably, technically a list is the collection of elements whereas an array is the variable that points to the list.
$#array contains the last element index of the array. Since arrays are zero-indexed, the size of the array is $#array + 1.
Negative array indices start from the end of the list, so the last element can be accessed as $array[-1].
An empty list is represented by (), and an empty array can be initialised as my @array = ();
List ranges can be generated using .., e.g. (1..10) will include the numbers from 1 to 10.
The qw function allows a list to be generated from elements separated by whitespace, without the need for quotes or commas, e.g. @array = qw(a b c d).
Array items can either be a scalar value, or a reference to another list. If you use an array in a list (e.g. @list = (@array)), it will be expanded to a scalar value rather than being a reference to the array.

Scoping hurts performance (but probably improves readability and keeps variables scoped)

Perl Optimisization Tidbits - some surprising results (Perl 5.22, so may no longer be relevant)
Modern Approaches to Ancient Perls