14_build.pl

package Starship;
use Moose;

has 'captain', is => 'ro', isa => 'Str', required => 1;
has 'crew', is => 'rw', isa => 'ArrayRef[Str]', required => 1;

sub BUILD {
    my $self = shift;
    if ( $self->captain ~~ $self->crew ) {
        my $captain = $self->captain;
        die "Validation Error: Cannot use $captain for both Captain and Crew";
    }
}


package Person;
use Moose;

has 'name', is => 'ro', isa => 'Str', required => 1;

around BUILDARGS => sub {
    my $orig   = shift;
    my $class  = shift;
    my @params = @_;

    # Sole parameter that is not a ref
    # is considered the name
    if ( ( @params == 1 ) && ( ! ref $params[0] ) ) {
        return $class->$orig( name => $params[0] );
    } else {
        return $class->$orig ( @params );
    }
};  # Watch the semicolon


package main;

# Validation error
my $enterprise = Starship->new( {
        captain => 'James T Kirk',
        crew    => ['Dr. McCoy', 'Scott', 'Lt. Uhura' ],
});