How to handle HTTP errors and exceptions with LWP in Perl?

When working with LWP::UserAgent in Perl to perform HTTP requests, it's important to handle HTTP errors and exceptions properly to make your code robust. LWP provides a rich set of tools to check HTTP responses and detect errors such as connection failures, timeouts, and HTTP error status codes (like 404 or 500).

Understanding HTTP Error Handling in LWP

LWP follows the "TMTOWTDI" ("There's More Than One Way To Do It") philosophy, giving multiple ways to handle errors:

• Checking the response's success status: The LWP::UserAgent method request returns an HTTP::Response object. You check whether the request succeeded using $response->is_success.
• Checking HTTP status codes: You can check $response->code and $response->message to understand the type of error.
• Handling network or protocol errors: If the request fails before getting an HTTP response (e.g., network down), you get an error string from $response->status_line.

It’s important to always test the success status so your program can gracefully handle failures.

Common Pitfalls

• Using get in scalar context returns the content even if the server returned an error status. Always check the response object for success.
• Ignoring redirects (3xx) - LWP usually handles some redirects automatically, but be aware of that.
• Not checking network errors which can cause your program to silently proceed with undefined or misleading data.

Example: Handling HTTP Errors with LWP

The following example demonstrates how to use LWP::UserAgent to make a GET request and handle errors properly:

use strict;
use warnings;
use LWP::UserAgent;

# Create a new UserAgent
my $ua = LWP::UserAgent->new(
    agent => 'PerlLWP/1.0',
    timeout => 10,
);

# URL to fetch
my $url = 'http://example.com/nonexistent';

# Perform the HTTP GET request
my $response = $ua->get($url);

# Check if the request was successful
if ($response->is_success) {
    print "Success! Content received:\n";
    print $response->decoded_content;  # Decode using charset if available
} else {
    # Handle errors - print status code and message
    warn "HTTP request failed:\n";
    warn "  Status code  : " . $response->code . "\n";
    warn "  Message      : " . $response->message . "\n";
    warn "  Status line  : " . $response->status_line . "\n";

    # You can add custom logic based on error codes:
    if ($response->code == 404) {
        warn "Resource not found (404).\n";
    } elsif ($response->code >= 500) {
        warn "Server error, try again later.\n";
    }
}

Explanation

• LWP::UserAgent->new instantiates an agent with a 10-second timeout and a custom user agent string.
• $ua->get($url) returns an HTTP::Response object, regardless of success or failure.
• $response->is_success returns true if the HTTP status code is in the 2xx or successful range.
• If the request failed, you output the code (e.g. 404), message ("Not Found"), and the full status line.
• Based on the status code, you can trigger additional logic like retries or fallbacks.

Version Notes and Enhancements

• LWP::UserAgent and HTTP::Response have been stable for many Perl 5 versions (5.8+).
• $response->decoded_content uses the content encoding from the server and requires at least Perl 5.8 and LWP with Encode support.
• You can use eval blocks and local callbacks for more advanced error and exception trapping, but for most cases, checking is_success is sufficient.

By routinely checking HTTP response status and handling errors explicitly, your LWP-based Perl scripts become more reliable and easier to debug.