Multi-line Strings | Here Document in Perl

In Perl, multi-line strings can be represented in a variety of ways, but one of the most idiomatic and flexible methods is the "here document" syntax, often referred to as a "heredoc". A heredoc allows you to define a string across multiple lines without needing to constantly break and concatenate the string, making it an ideal solution for long strings, configuration data, or multi-line output. This tutorial will guide you on how to use the heredoc syntax in Perl.

1. Basic Here Document Syntax

A heredoc starts with << followed by an identifier of your choosing. The string then starts on the next line and continues until the identifier is reached again.

my $string = <<END_TEXT;
This is a multi-line string.
You can add as many lines as you want.
The string ends when the identifier is reached again.
END_TEXT

print $string;

The output will be:

This is a multi-line string.
You can add as many lines as you want.
The string ends when the identifier is reached again.

2. Using Variables in a Heredoc

You can use variables inside a heredoc, and they will be interpolated:

my $name = "Alice";
my $message = <<GREETING;
Hello, $name!
Welcome to the world of Perl heredocs.
GREETING

print $message;

The output:

Hello, Alice!
Welcome to the world of Perl heredocs.

3. Suppressing Variable Interpolation

If you want the content inside the heredoc to be treated as a single-quoted string (where variable interpolation does not occur), you can enclose the identifier in single quotes:

my $name = "Alice";
my $message = <<'GREETING';
Hello, $name!
Variables won't be interpolated here.
GREETING

print $message;

The output:

Hello, $name!
Variables won't be interpolated here.

4. Indented Heredocs (since Perl 5.26)

With Perl 5.26 and newer, you can use the <<~ syntax to create a heredoc with indented content:

my $string = <<~'TEXT';
    This is an indented heredoc.
    Each line's content will be shifted to the left to remove the leading whitespace.
TEXT

print $string;

Output:

This is an indented heredoc.
Each line's content will be shifted to the left to remove the leading whitespace.

5. Practical Use Cases

Heredocs are useful for:

SQL queries
Configuration data
HTML or XML content
Any situation where a multi-line string is easier to read and manage than concatenated single-line strings.

6. Tips and Gotchas

The ending identifier must start at the beginning of the line (no whitespace before it) and be followed immediately by a newline or a semicolon.
It's common to use all-uppercase identifiers for heredocs, but it's not a strict requirement.
Since the heredoc ending identifier must appear by itself on its own line, it's important to ensure that it doesn't accidentally appear within the text of the heredoc.

Conclusion

The heredoc syntax in Perl provides an elegant way to handle multi-line strings, making your scripts cleaner and more readable. By understanding the nuances of heredocs and how to leverage them effectively, you can simplify the way you handle large strings in your Perl code.