I'll also point out a technique you could use. Instead of FooFactory introspecting class Foo, you could have class Foo pass FooFactory all the information it needs. One simple way to do that is write a wrapper for has...

#!/usr/bin/env perl

use v5.14;
use Data::Dumper;

package FooFactory {
    use Moo;
    has name => (is => "ro", required => 1);
    our @known_attributes;
    sub build_foo {
        my $self = shift;
        my %params = @_;
        my %attrs;
        for (@known_attributes) {
            $attrs{$_} = $params{$_} if exists $params{$_};
        }
        'Foo'->new(%attrs, maker => $self);
    }
}

package Foo {
   use Moo;
    sub my_has {
        my ($attr, %spec) = @_;
        # Inform FooFactory of the new attribute
        # Note we're ignoring init_arg for this simple example.
        push @FooFactory::known_attributes, $attr;
        has($attr, %spec);
    }
    my_has foo   => (is => "ro");
    my_has bar   => (is => "ro");
    my_has baz   => (is => "ro");
    my_has maker => (is => "ro");
}

my $factory = 'FooFactory'->new(name => "Acme Inc");
my $obj = $factory->build_foo(bar => 42);

print Dumper($obj);
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