What is the proper way to get the caller to our object creation (the object's client code) inside Mo/Moo/Moose's BUILD or BUILDARGS? I'm okay with getting a subclass.

From a quick glance of the Moo and Moose codebase, it doesn't seem like Moo/Moose provides a utility routine for this. A quick search on CPAN also doesn't yield anything yet.

Example:

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
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In principle it should be easy enough to loop over the caller stack and use the first non-Moo* stuff.