For a solution perhaps more sophisticated that the classic negate-$x-each-iteration perhaps it would be illuminating to look at Easy binary toggle at fixed intervals, or perhaps Flipper, and of course, merlyn has probably written an article about this :)

IMHO the simplest is:

d:\>perl -e "print +($a += 1)  %= 2 for 1..10"
1010101010
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