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).

G'day Neil,

Without seeing your module code, I can't say specifically what's wrong. I suspect your has functions have one or more of required, lazy or default incorrectly set up. See Moose - EXPORTED FUNCTIONS for details. The following works for me.

Moose module: pm_moose_default_params.pm

package pm_moose_default_params;

use Moose;

has colour => (
    is       => 'rw',
    isa      => 'Str',
    required => 0,
    lazy     => 1,
    default  => 'red',
);

has shape => (
    is       => 'rw',
    isa      => 'Str',
    required => 0,
    lazy     => 1,
    default  => 'circle',
);

no Moose;
__PACKAGE__->meta->make_immutable;
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Calling script: pm_moose_default_params.pl

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

use Getopt::Long;

use pm_moose_default_params;

my $colour = '';
my $shape = '';

GetOptions('colour=s' => \$colour, 'shape=s' => \$shape);

say "Option --colour = $colour";
say "Option --shape = $shape";

my %param;
$param{colour} = $colour if $colour;
$param{shape} = $shape if $shape;

say "Param $_ = $param{$_}" for keys %param;

my $object = pm_moose_default_params::->new(%param);

say 'Moose colour = ', $object->colour;
say 'Moose shape = ', $object->shape;
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Some sample runs:

$ pm_moose_default_params.pl
Option --colour = 
Option --shape = 
Moose colour = red
Moose shape = circle

$ pm_moose_default_params.pl --colour=blue
Option --colour = blue
Option --shape = 
Param colour = blue
Moose colour = blue
Moose shape = circle

$ pm_moose_default_params.pl --shape=square
Option --colour = 
Option --shape = square
Param shape = square
Moose colour = red
Moose shape = square

$ pm_moose_default_params.pl --colour=blue --shape=square
Option --colour = blue
Option --shape = square
Param colour = blue
Param shape = square
Moose colour = blue
Moose shape = square
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That's a lot of work for the calling code. I'm not a Moose expert either but I'd use a BUILDARGS method, that's significantly shorter and cleaner already:

sub BUILDARGS {
    my ($class, %args) = @_;
    defined $args{colour} and '' ne $args{colour} or $args{colour} = '
+#123';                                      
    return \%args;
}
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