Add ability to user provide default values to moose attributes via import

t.pl

#!/usr/bin/env perl 

use v5.14;
use strict;
use warnings;

package T {
    use Moose;

    use vars qw(%defaults);

    sub import {
        my ( $class, %opts ) = @_;

        $defaults{a} = $opts{default_a} if exists $opts{default_a};
        $defaults{b} = $opts{default_b} if exists $opts{default_b};
    }

    has a => (
        is  => 'rw',
        isa => 'Int',
        ( exists $defaults{a} ? ( default => $defaults{a} ) : () ),
    );

    has b => (
        is  => 'rw',
        isa => 'Str',
        ( exists $defaults{b} ? ( default => $defaults{b} ) : () ),
    );
}

package main {
    use T default_a => 123, default_b => 'abc';

    my $t = T->new( b => 'def' );

    say $t->a;
    say $t->b;
}