This is posted in response to qball's question about alternating row colors. The sub &cycle (yes, I know there is a Tie::Cycle module) takes two arguments. The first is the number of times each element gets returned prior to moving to the next element. The second argument is a reference to an array that contains the elements to cycle through. The code below prints: 
red
red
white
white
blue
blue
red
red
white
white

[download]

Update: Yeah, I changed the sub name while I was posting it. Oops! Also, the comments about my second sanity check (array ref) are correct in that my error message doesn't match the actual check. My thought was that the check would simultaneously catch whether or not it was an array and whether it would have more than zero elements. So much for trying to golf validation :)

use warnings;
use strict;

my @colors = qw/ red white blue /;
my $rotate = cycle( 2, \@colors );

for ( 1..10 ) {
    print &$rotate . "\n";
}

sub cycle {
    my ( $toggle, $items ) = @_;
    my $error = '';
    if ( $toggle !~ /^\d+$/ or $toggle == 0 ) {
        $error = "The first argument to &alternate must be a positive 
+integer: $toggle\n";
    }
    if ( ! @$items ) {
        $error .= "The second argument to &alternate must be an array 
+reference.";
    }
    die $error if $error;
    my $count = 0;
    my $index = -1;
    return sub {
        $index += 1 if $count++ % $toggle == 0;
        $index = 0  if $index == @$items;
        $items->[ $index ];
    }
}

[download]

In reply to Simple Rotation by Ovid

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