package Class1;
use Moo;

has attr1 => (is => 'rw');

sub BUILD {
    no strict 'refs';

    my $self = shift;
    # XXX set default for attr1 depending on the caller
    unless (defined $self->attr1) {
        $self->attr1(${"$object_caller_package\::FOO"});
    }
}

package C2;
use Moo;
extends 'C1';

package main;

our $FOO = 42;
say C2->new->attr1; # prints 42