Here's a quick transformation of that into some code (not complete). Some issues I'm trying to think about now are:

When a value in the card is changed (for example, the x,y coordinates) it needs to be updated (perhaps I should get rid of this and handle it on the Tk/GUI end; possible advantage is that it would extend the module to work with other GUIs or even with text)

Hmm. . . actually, that idea would make the other issues I was working with go away too. Comments?

#!/usr/bin/perl

######################################
# cards.pm -- a perl module to allow #
# for the easy creation of solitare  #
# games (easily expanded to others)  #
######################################

package cards;

use strict;
use warnings;

######################################
# Card type and methods              #
######################################

sub new {
  # usage: $caller->new([\%attr])
  # keys for %attr: x_pos, y_pos,
  # suit, value, where, state,
  # gif, deck (for future use)

  my $proto = shift;
  my $self  = shift;
  my $class = ref($proto) || $proto;
  bless ($self, $class);
  return $self;
}

sub show {
  # usage: $caller->show([\%pos])
  # keys for %pos: x_pos, y_pos
  # return: hash with keys x_pos,
  # y_pos (of upper-left corner)

  my $self = shift;
  if (@_) {
    $self->{x_pos} = $_->{x_pos};
    $self->{y_pos} = $_->{y_pos};
  }
  return { x_pos => $self->{x_pos}, y_pos => $self->{y_pos} };
}

sub suit {
  # usage: $caller->suit([$suit])
  # $suit =~ /[CDHScdhs]/
  # return: suit of card

  my $self = shift;
  if (@_) { $self->{suit} = shift }
  return $self->{suit};
}

sub value {
  # usage: $caller->value([$val])
  # $val =~ /[1-13AKQJ]/
  # return: value of card

  my $self = shift;
  if (@_) { $self->{value} = shift }
  return $self->{value};
}

sub where {
  # usage: $caller->where([$pile])
  # $pile is of type Pile
  # return: Pile reference where
  # card is located

  my $self = shift;
  if (@_) { $self->{where} = shift }
  return $self->{where};
}

sub state {
  # usage: $caller->state([$state])
  # $state =~ (up)|(down)
  # return: returns state of card

  my $self = shift;
  if (@_) { $self->{state} = shift }
  return $self->{state};
}


######################################
# "Friend" functions for rules       #
######################################

sub IsAlternatingColor {
  # usage: IsAlternatingColor(@cards)
  # where for all i less than $#cards
  # $cards[i] is of type Card
  # return: true or false (1 or 0)

  my $cards = shift;

  # A zero represents black, a 1 red; orignially set
  # $old_color to opposite of that of the first card
  my $old_color = $cards->[0]->suit() =~ /CScs/ ? 1 : 0;
  my $cur_color;

  for (@$cards) {
    $cur_color = $_->suit() =~ /CScs/ ? 0 : 1;
    return 0 if $cur_color = $old_color;
  }
  return 1;
}

sub IsSameColor {
  # usage: IsSameColor(@cards)
  # where for all i less than $#cards
  # $cards[i] is of type Card
  # return: true or false (1 or 0)

  my $cards = shift;

  # A zero represents black, a 1 red; orignially set
  # $old_color to the same of that of the first card
  my $old_color = $cards->[0]->suit() =~ /CScs/ ? 0 : 1;
  my $cur_color;

  for (@$cards) {
    $cur_color = $_->suit() =~ /CScs/ ? 0 : 1;
    return 0 if $cur_color != $old_color;
  }
  return 1;
}
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