# In the configuration section...
my @Toggles = ('on', 'off');

# In some loop in the code...
$Toggles[$flip = !$flip]

##</code><code>##

my @BGColors = ('#dddddd', '#ffffff');

for (1..10){ # As you loop through a database query result or something
  print qq{
    <tr bgcolor="$BGColors[$flip = !$flip]">
      <td>$_</td>
    </tr>
  };
}

##</code><code>##

# To cycle through any number of elements...
my @Cheers = ('hip', 'hip', 'hoo', 'ray');
my $flip = -1;
for (1..12){
  print "$Cheers[$flip = ++$flip % 4]\n";
}