date-coercion.pl

use strict;
use warnings;

{
	package Event;
	use Moose;
	use Types::Standard qw( Str );
	use Types::XSD qw( Date );
	
	has start_date => (
		is       => 'ro',
		isa      => Date->plus_coercions(Str, \&_format_as_iso8601),
		coerce   => 1,
		required => 1,
	);
	
	sub _format_as_iso8601
	{
		my $date = @_ ? shift : $_;
		my ($d, $m, $y) = split m{/}, $date;
		sprintf('%04d-%02d-%02d', $y, $m, $d);
	}
}

print Event->new(start_date => '1/6/1980')->dump;