4.1 Program Structure

Perl is a particularly forgiving language, as far as program layout goes. There are no rules about indentation, newlines, etc. Most lines end with semicolons, but not everything has to. Most things don't have to be declared, except for a couple of things that do. Here are the bare essentials:

Whitespace

Whitespace is required only between items that would otherwise be confused as a single term. All types of whitespace - spaces, tabs, newlines, etc. - are equivalent in this context. A comment counts as whitespace. Different types of whitespace are distinguishable within quoted strings, formats, and certain line-oriented forms of quoting. For example, in a quoted string, a newline, a space, and a tab are interpreted as unique characters.

Semicolons

Every simple statement must end with a semicolon. Compound statements contain brace-delimited blocks of other statements and do not require terminating semicolons after the ending brace. A final simple statement in a block also does not require a semicolon.

Declarations

Only subroutines and report formats need to be explicitly declared. All other user-created objects are automatically created with a null or 0 value unless they are defined by some explicit operation such as assignment. The -w command-line switch will warn you about using undefined values.

You may force yourself to declare your variables by including the use strict pragma in your programs (see Chapter 8, Standard Modules, for more information on pragmas and strict in particular). This makes it an error to not explicitly declare your variables.

Comments and documentation

Comments within a program are indicated by a pound sign (#). Everything following a pound sign to the end of the line is interpreted as a comment.

Lines starting with = are interpreted as the start of a section of embedded documentation (pod), and all subsequent lines until the next =cut are ignored by the compiler. See Section 4.11, "Pod" later in this chapter for more information on pod format.