So, if I'm understanding your problem correctly, you are building your object with undefined attributes and even if you have defaults set for the attributes, they still get set to undef, right?

I deal with this by using MooseX::UndefTolerant, whose brief description is: "Make your attribute(s) tolerant to undef initialization". One more thing: for this to work, it seems that you need to make your attribute lazy. Below is an example.

test_undef.pm:

package test_undef;
use Moose;
use MooseX::UndefTolerant;

has 'language' => (
    is      => 'rw',
    isa     => 'Str',
    default => 'Perl',
    lazy    => 1,
);

no Moose;
__PACKAGE__->meta->make_immutable;
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test_undef.pl:

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use test_undef;
use Getopt::Long;

my $language;
GetOptions(
    "language=s" => \$language,
);
my $test = test_undef->new(
    'language' => $language,
);

say $test->language;
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If I call ./test_undef.pl without the --language option (or w/o defining language any other way), the output is: Perl (the default I set for the language attribute).