How to use Moo lazy_build for attribute defaults in Perl?

The issue arises because lazy_build is not a core feature of Moo itself; it comes from the helper module MooX::LazyBuild. Without loading this module, the clear_config method is not auto-generated, resulting in the runtime error Can't locate object method "clear_config".

To safely use a lazy builder with Moo without additional modules, you can explicitly specify lazy => 1, provide a builder, and optionally a clearer method yourself. This manual approach replicates what lazy_build automates.

Working Example With Manual Lazy Builder Setup in Moo

use strict;
use warnings;
use Data::Dumper;

{
    package MyClass;
    use Moo;

    # Define 'config' as lazy with manual builder and clearer
    has 'config' => (
        is      => 'rw',
        lazy    => 1,
        builder => '_build_config',
        clearer => 'clear_config',
    );

    # Builder method must be named _build_config
    sub _build_config {
        print "Building config attribute...\n";
        return { key => "value", timestamp => time };
    }
}

# Main script demonstration
my $obj = MyClass->new();

print "Object created, config not built yet.\n\n";

print "Accessing config first time:\n";
my $conf = $obj->config;
print Dumper($conf);

print "Accessing config second time (cached, no rebuild):\n";
print Dumper($obj->config);

print "Clearing config cache...\n";
$obj->clear_config;

print "Accessing config after clearing (rebuild occurs):\n";
print Dumper($obj->config);

Explanation

• has defines the attribute config with three options:
  • lazy => 1 delays building the attribute until first access.
  • builder => '_build_config' specifies the method to call for the default value.
  • clearer => 'clear_config' creates a method to clear the cached value, forcing rebuild on next access.
• Without MooX::LazyBuild, you must manually set these options yourself instead of lazy_build => 1.
• This approach is fully supported in core Moo and avoids runtime errors.
• Data::Dumper is used to print the attribute structure, and is properly imported.

Common Pitfalls

• Assuming lazy_build is built into Moo causes missing clearer methods.
• Builder methods must be named exactly as given in the builder option (underscore prefix is common).
• Not providing a clearer method means you cannot clear the cached value for rebuild.
• Always import or fully qualify Dumper to avoid runtime errors.

This example is runnable as-is with perl - and meets your constraints—no external modules, no file or network I/O, and runs instantly. Using manual lazy builder setup with Moo ensures compatibility and control.