How to parse JSON file and extract specific fields in Perl?

Below is a comprehensive guide on how to parse JSON and extract specific fields, along with a runnable Perl example you can execute directly. This example illustrates key Perl concepts like sigils, context, and typical gotchas.

Step-by-step approach

• Read JSON content: Often from a file or string.
• Decode JSON: Convert JSON text to Perl data structures (using decode_json).
• Access fields: Use Perl hashes or arrays to extract specific values.
• Handle errors: Use eval or check for decoding failures.

Perl Specifics

• $ sigil indicates scalar (string, number, ref).
• @ sigil for arrays.
• % sigil for hashes.
• JSON decoding returns references usually (e.g. hashref).
• Context matters: accessing $data->{key} vs @{ $data->{array} }.
• Perl's TMTOWTDI ("There's more than one way to do it") applies; several JSON modules exist, but JSON::PP is core and reliable.

Example: Parse JSON string and extract fields

This example demonstrates how to parse a JSON string representing a user list, then extracts and prints the name and email of each user.

use strict;
use warnings;
use JSON::PP;  # Core from Perl 5.14+, pure Perl fallback

# Sample JSON text (could be read from a file)
my $json_text = q|{
  "users": [
    { "name": "Alice", "email": "alice@example.com", "age": 30 },
    { "name": "Bob", "email": "bob@example.com", "age": 25 },
    { "name": "Carol", "email": "carol@example.com", "age": 27 }
  ]
}|;

# Decode JSON string into Perl data structure (hashref)
my $data = eval { decode_json($json_text) };
if ($@) {
    die "Failed to decode JSON: $@";
}

# Access the arrayref of users
my $users = $data->{users};

# Print extracted fields
foreach my $user (@$users) {
    # $user is a hashref for each user
    my $name  = $user->{name}  // 'Unknown';
    my $email = $user->{email} // 'No email';
    print "Name: $name, Email: $email\n";
}

Explanation

We use decode_json from JSON::PP to convert the JSON string into a Perl hash reference. The key users holds an array reference, so accessing $data->{users} provides that array reference. Iterating through @$users retrieves each user's hashref, letting us directly access {name} and {email}.

The use of the defined-or operator // ensures we have default values when keys are missing.

Common Pitfalls

• Failing to decode JSON and not checking for errors (use eval or error checking).
• Misunderstanding references and forgetting to dereference arrays/hashes.
• Assuming JSON keys are always present (handle undefined values gracefully).
• Confusing scalar vs list context when extracting data.
• Using non-core JSON modules without installation, which can cause portability issues.

Notes on Version and Modules

• JSON::PP is a pure-Perl module bundled with Perl 5.14 and newer, making it great for portability.
• Alternatively, JSON or JSON::XS can be faster but may require CPAN installation.
• The example works on Perl 5.14+ without external dependencies.

This example should serve as a base for extracting fields from JSON. For files, simply replace the hardcoded string and read the JSON content from the file like this:

open my $fh, "<", "file.json" or die $!;
my $json_text = do { local $/; <$fh> };
close $fh;

With this setup, you can extend parsing to more complex nested data and various field extractions with confidence.