Scalars, Arrays, and Hashes

Perl has exactly three built-in data structure types, and it tells them apart with a sigil, a leading punctuation character on every variable name: a dollar sign $ marks a scalar, which holds a single value such as a number, string, or reference; an at sign @ marks an array, an ordered, indexed list of scalars; and a percent sign % marks a hash, an unordered collection of key-value pairs. Crucially, the sigil describes how many values you are accessing right now rather than the variable's declared type, so @fruits refers to the whole array but $fruits[0] refers to a single scalar element pulled out of that same array, and this consistent rule is one of the first things that trips up newcomers coming from languages with a single, uniform indexing syntax.

A scalar variable, declared with my $name = 'Alice';, holds exactly one piece of data at a time, and Perl does not require you to declare whether that data is a number or a string; the same scalar can hold 42, 'hello', a floating-point number like 3.14, or even a reference to an array or hash. Perl automatically converts between numbers and strings depending on context, so my $count = '5' + 3; yields 8 because the + operator forces numeric context, while my $greeting = 'Item ' . 5; yields the string 'Item 5' because the . concatenation operator forces string context; this implicit conversion is powerful but is also why use warnings; is valuable, since it will flag things like adding a non-numeric string to a number.

An array, declared as my @colors = ('red', 'green', 'blue');, is a zero-indexed ordered list, so $colors[0] is 'red' and $colors[-1] conveniently returns the last element, 'blue', without needing to know the array's length. Arrays grow and shrink dynamically: push @colors, 'yellow'; appends an element, pop @colors; removes and returns the last one, and scalar @colors (or simply using @colors in a place that expects a number) tells you how many elements it holds. A common beginner mistake is writing $colors instead of $colors[0] when trying to grab the first element, since Perl will not automatically assume you meant the first element; $colors alone actually refers to a completely separate, unrelated scalar variable in Perl's namespace.

A hash, declared as my %ages = (Alice => 30, Bob => 25);, stores unordered key-value pairs where every key is a unique string and every value is a scalar, so $ages{Alice} retrieves 30 using curly braces rather than the square brackets used for arrays. The keys %ages function returns a list of all the keys, values %ages returns a list of all the values in the same corresponding order, and exists $ages{Carol} lets you safely check whether a key is present before accessing it, which avoids the subtle bug of auto-vivifying a new key with an undef value just by testing if ($ages{Carol}). Because hashes have no guaranteed order, code that needs a predictable iteration order should explicitly sort the keys, for example for my $name (sort keys %ages) { ... }, rather than relying on the order keys happen to come back in.