order: quantity color item
quantity: /\d+/
color: # what should this be?
item: 'pen' | 'pencil'

##</code><code>##

my @colors = ('red', 'green', 'blue');

##</code><code>##

my @colors = ('cyan', 'magenta', 'burnt umber');

##</code><code>##

sub is_valid_color
{
  my ($color) = @_;

  foreach (@colors)
  {
    return 1 if $color eq $_;
  }
  return undef;
}