Perl quick reference

The smallest Perl program, use v5.40, and printing with say and print.

The three kinds of variable — $scalar, @array, %hash — my, undef, and numbers vs strings.

if / elsif / unless, for over a list or range, while, statement modifiers, and next / last.

Define subroutines with sub and signatures, default arguments, returning lists, the older @_ style, and anonymous subs.

push, sort, map and grep, slices, exists, // for defaults, and counting with a hash.

Build hashes of arrays and lists of hashes with references: [...], {...}, -> and postfix dereference ->@*.

String functions, printf, matching with =~, capture groups, /g, and substitution with s///r.

Read what you type in the IDE's Input tab with <STDIN> and chomp.

Open project files with three-argument open and or die, read line by line, and write temporary files under /tmp.

Put subs in a .pm module, load it with use lib and use, and use core modules such as List::Util.

What Run does with your project, the time limit, input, tests, and what a run cannot do.

What Global symbol requires explicit package name, Use of uninitialized value, isn't numeric, Can't locate and strict refs mean and how to fix them.

Where to read more: perlintro, the function and operator references, the regex tutorial and perldiag.