my ($class, %flip_class) = qw(odd odd even even odd);

while (...) {
	$class = $flip_class{$class};
	...
}

##</code><code>##

my ($class, %flip_class) = qw(odd even)[0,1,0,0,1];

while (...) {
    $class = $flip_class{$class};
    ...
}

##</code><code>##

my $class = 'odd';

while (...) {
	($class) = grep {!/$class/} qw(odd even);
	...
}