How to escape special characters when encoding JSON in Perl?

When encoding JSON in Perl, properly escaping special characters is crucial to produce valid JSON output that can be safely parsed by JSON parsers in other languages or environments. Special characters in JSON strings include quotes, backslashes, control characters, and certain Unicode characters that must be escaped to avoid syntax errors or unexpected behavior.

Using a JSON Module for Proper Escaping

Instead of manually escaping special characters — which can be error-prone and cumbersome — it's strongly recommended to use a dedicated JSON module that handles all escaping correctly and efficiently. The Perl core ecosystem has several JSON libraries, but JSON::PP is a pure-Perl module included in the core since Perl 5.14, and it handles escaping according to the JSON specification (RFC 8259).

These modules automatically escape characters like:

• Double quotes (") as \"
• Backslash (\) as \\
• Control characters like newline (\n) and tab (\t)
• Unicode characters outside the ASCII range (using UTF-8 encoding or \uXXXX escapes)

Manually escaping JSON strings is discouraged because the rules vary based on context and Unicode support, and the modules optimize and conform to standards.

Runnable Perl Example Using JSON::PP

use strict;
use warnings;
use JSON::PP;

# Sample data with special characters
my $data = {
    message => qq{He said, "Hello\nWorld!" and smiled.},
    path    => q{C:\Users\Test},
    control => "\x01\x02",    # control chars
};

# Create a JSON::PP object
my $json = JSON::PP->new->utf8->pretty;

# Encode the data to JSON string
my $json_text = $json->encode($data);

print "JSON encoded string with escaped special chars:\n$json_text\n";

This outputs:

JSON encoded string with escaped special chars:
{
   "control" : "\u0001\u0002",
   "message" : "He said, \"Hello\nWorld!\" and smiled.",
   "path" : "C:\\Users\\Test"
}

Key Concepts Explained

• Sigils: Scalars hold strings or numbers (e.g. $data), arrays use @, and hashes use %.
• Context: The JSON encoder inspects your Perl data structure (usually a hash or array reference) and serializes it appropriately.
• TMTOWTDI: Perl’s "There's More Than One Way To Do It" philosophy means you can also use other JSON modules like JSON XS or Cpanel::JSON::XS, but JSON::PP is core and sufficient for most cases.

Common Pitfalls

• Manually escaping JSON strings often leads to errors, such as missing some characters or mishandling Unicode.
• Ensure the JSON encoder is set to output UTF-8 if you have non-ASCII characters. Use utf8 or encode_utf8 as shown.
• Don’t confuse escaping JSON with escaping for other formats like HTML or URLs.
• Using print with JSON encoded strings that contain UTF-8 data requires the output handle to be set to UTF-8 if you want proper display (outside the scope of this example).

In summary, always use a standard Perl JSON module like JSON::PP for escaping special characters during JSON encoding. It ensures correctness, readability, and portability of your JSON data.