use Attribute::Property;

{
    package Param::function;
    sub new : New;

    # XXX - Both need anchors?
    sub type  : Property { /Order|Return/ }
    sub month : Property { grep /$_[1]/, 1 .. 12 }
}
sub function { 
    my $self = shift;
    ...
}

##</code><code>##

my %params = (
    type  => qr/Order|Return/,
    month => sub { my $month = shift; return grep { /$month/ } (1 .. 12) }
);
my $param = Param::function->new(\%params);

$param->type = 'Return';
$param->month = 3;

print $param->type;  # prints 'Return'
print $param->month; # prints 3

# the following two method calls will fail (croaking)
$param->type = 'Ovid';
$param->month = 13;

##</code><code>##

my $param = @_ == 1 ? shift : Param::function->new({ @_ });