A combination of both insensate's and Jasper's gems.

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perl -pe '$_ x=!$$_++'
[download]

Update: This is, unfortunately, version specific. It works on 5.8 but not on earlier versions. See the short discussion starting with jmcnamara's reply to insensate. Another variation which shouldn't be version specific and which ties Jasper's:

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perl -pe '$_ x=!${_.$_}++'
[download]

-sauoq
"My two cents aren't worth a dime.";