if( !$x && !$y &&  $z or
    !$x &&  $y && !$z or
     $x && !$y && !$z    )
{
    print "Exactly one...";
}
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I don't think I was clear. I should have added: "How do you expand that for 7, 42, or 105 variables?" It can be done mechanically, fairly easily, but I wouldn't want to end up with an arrangement of 105 x 105, nor would I want to maintain it.

One might argue that getting from K to K+1 or K-1 variables is also easy (and it is). But I still wouldn't want to deal with it when I could use a list syntax instead.

Cheers,