%states = ( 
    'default'     => \&front_page,
    'Shirt'       => \&shirt,
    'Sweater'     => \&sweater,                                      
);

$current_screen = param(".state") || "default";                      
die "No screen for $current_screen" unless states{$current_screen}; 

# Generate the current page.

standard_header();                                                   

#how does this while loop work?
while (my($screen_name, $function) = each %states) {                 
    $function->($screen_name eq $current_screen);
}
standard_footer();
exit;

#############################                                        
# subroutines for each screen                                        
#############################

# The default page.
sub front_page {  
    my $active = shift;     
    return unless $active; 

    # how do the above 2 lines work?

    print "<H1>Hi!</H1>\n";
    print "Welcome to our Shirt Shop!  Please make your selection from "; 
    print "the menu below.\n";

    shop_menu();
}

# Page to order a shirt from.
sub shirt {
    my $active = shift; #what does this line do?
    my @sizes  = qw(XL L M S);
    my @colors = qw(Black White);

    my ($size, $color, $count) =
      (param("shirt_size"), param("shirt_color"), param("shirt_count"));

... yada yada