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    return ($args->{source});
}

sub BUILD {                     # runs AFTER all attributes are set; parent→child order
    my ($self, $args) = @_;
    die "invalid" unless length $args->{source};
}
# DEMOLISH: child→parent order. Never override DESTROY directly.
```

Do NOT call `SUPER::BUILD` manually – Moo handles the chain.

---

## Pattern 10 – Strict Constructor

```perl
package StrictThing;
use Moo;
use MooX::StrictConstructor;
has size => (is => 'rw');

.claude/skills/perl-moo/SKILL.md  view on Meta::CPAN

---

## Common Pitfalls

- `default => []` → **shared state bug**. Always `default => sub { [] }`.
- `extends 'A'; extends 'B'` → replaces, does NOT add B to A. Use `extends 'A', 'B'`.
- Imports after `use Moo::Role` are **composed into consumers** as methods.
- `namespace::autoclean` < 0.16 inflates Moo classes to Moose unexpectedly.
- `trigger` does NOT receive old value (unlike Moose).
- `Sub::HandlesVia` must be loaded *after* `use Moo`.
- `BUILD` chain is automatic; calling `SUPER::BUILD` manually breaks it.
- Never override `DESTROY`; use `DEMOLISH`.