{
    Readonly my $spec => {
        size  => { type => SCALAR },
        color => { type => SCALAR,
                   regex => qr/^(?:red|green|blue)$/i },
    };
    sub item_by_size_and_color {
        my %p = validate( @_, $spec );

        ...
    }
}

##</code><code>##

  { size  => SCALAR_TYPE,
    color => RGB_COLOR_TYPE( default => 'blue' ),
    ...
   }