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;