How to set custom headers in LWP::UserAgent requests in Perl?

Setting Custom Headers with LWP::UserAgent in Perl

When working with HTTP requests in Perl, the LWP::UserAgent module is a powerful and flexible choice. Sometimes, you need to add custom headers to your requests—for example, to specify an API key, user agent string, or other HTTP headers required by the server.

Here’s how you can set custom HTTP headers with LWP::UserAgent:

• Create an LWP::UserAgent object.
• Create an HTTP::Request object for the HTTP method & URL you want.
• Use the header method on the request object to set custom headers.
• Call request on the user agent with your modified request object.

Important Perl Concepts

Note that LWP::UserAgent and HTTP::Request come from separate classes, so you build the request first and then hand it to the user agent to perform. Headers are set as key-value pairs in the HTTP::Request object.

Perl’s flexible context and sigils mean you can easily manipulate and pass these objects around. Unlike some other HTTP clients, LWP::UserAgent doesn’t require TLS configuration for simple use, but always consider HTTPS support in real apps.

Runnable Example

use strict;
use warnings;
use LWP::UserAgent;
use HTTP::Request;

# Create a user agent
my $ua = LWP::UserAgent->new;

# Target URL (httpbin echoes back request data - great for testing)
my $url = 'https://httpbin.org/get';

# Create an HTTP GET request
my $req = HTTP::Request->new(GET => $url);

# Set custom headers
$req->header('X-Custom-Header' => 'MyValue');
$req->header('User-Agent' => 'MyLWPClient/1.0');
$req->header('Authorization' => 'Bearer ABC123XYZ');

# Perform the request
my $res = $ua->request($req);

# Check and print response
if ($res->is_success) {
    print "Response (first 500 chars):\n";
    print substr($res->decoded_content, 0, 500), "\n";
} else {
    print "HTTP Request failed: ", $res->status_line, "\n";
}

What This Code Does

• Initializes a new LWP::UserAgent object.
• Makes a GET request with three custom headers: X-Custom-Header, User-Agent, and Authorization.
• Sends the request, then prints either the first 500 characters of the response body or an error message.

Key Points & Common Pitfalls

• Headers must be set on the HTTP::Request object, not directly on LWP::UserAgent.
• Be mindful of header names: header names are case-insensitive, but the standard style capitalizes each word and uses hyphens.
• User-Agent can be overridden as a header this way; otherwise, LWP::UserAgent uses a default string.
• When setting Authorization, ensure your credentials follow the protocol (e.g., Basic, Bearer). This example uses a Bearer token format as a mock.
• For POST or PUT requests, you can add content (body) with $req->content or $req->content(\$scalar).
• Perl’s flexibility allows multiple calls to $req->header() to set many headers conveniently.

Version Notes

LWP::UserAgent and HTTP::Request use standard interfaces that have been stable across Perl 5.10+ releases. No special version handling is needed here. This approach works well on all modern Perl installations.

In conclusion, setting custom HTTP headers in LWP::UserAgent requires creating an HTTP::Request object, adding headers to it with the header method, and sending the request through your user agent. This pattern supports TMTOWTDI ("There’s more than one way to do it") by allowing full control over HTTP request details.