This might not be quite right but might offer a jump off. Update: Eliya's answer and example is better.

use warnings;
use strict;

my $hai = O::HAI->new({ arg => "KTHX" });

BEGIN {
    package O::HAI;
    use B::Deparse;
    use Moose;

    has "arg" => isa => "Str", is => "ro";

    sub BUILD {
        my ( $package, $filename, $line, $subroutine ) = caller(4);
        my $sub_ref =  do {
             no strict "refs";
             \&{join("::",$package,$subroutine)};
        };
        my $deparse = B::Deparse->new("-p", "-sC");
        my $caller = $deparse->coderef2text($sub_ref);
        print $caller, $/;
    }
}
[download]

{
    package Moose::Object;
    use warnings;
    use strict 'refs';
    (my $class = shift(@_));
    (my $real_class = (&Scalar::Util::blessed($class) || $class));
    (my $params = $real_class->BUILDARGS(@_));
    return('Class::MOP::Class'->initialize($real_class)->new_object($p
+arams));
}
[download]