I got tired of doing this all the time, so I created Tie::Cycle. Every time I use the tied scalar, it advances itself along its possible values:

use Tie::Cycle;

tie my $cycle, 'Tie::Cycle', [ qw( even odd  ) ];

foreach my $row ( 0 .. 6 )
    {
    print qq|<tr class="$cycle">...</tr>\n|;
    }
[download]

In the output, it flip flops between "odd" and "even".

<tr class="even">...</tr>
<tr class="odd">...</tr>
<tr class="even">...</tr>
<tr class="odd">...</tr>
<tr class="even">...</tr>
<tr class="odd">...</tr>
<tr class="even">...</tr>
[download]

It gets better when you want to cycle through more values. Just add them to the call to tie:

use Tie::Cycle;

tie my $cycle, 'Tie::Cycle', [ qw( red white blue ) ];

foreach my $row ( 0 .. 6 )
    {
    print qq|<tr class="$cycle">...</tr>\n|;
    }
[download]