use Foo;

my ($bar) =  @ARGV;
my $foo = Foo->new( bar => $bar );

##</code><code>##

package Foo;
use Moose;

has bar => (is => "ro", default => "whatever");

##</code><code>##

package Foo;
use Moose;

has a => (is => "rw");

sub BUILD {
  my($this, $args)=@_;

  $this->bar("whatever") unless defined $args->{bar};
}