Maybe you could explain your interpretation of the problem, because it seems ambiguous to me. 18 characters, and it does some stuff: 
sub X {
(1<<pop)&(pop|pop)
}

[download]
Here I'm presuming $a and $b are two values, and that a third parameter $c indicates which bit is required, such that if $c were 4, the 4th bit was extracted (24 or 16) and returned as is, with the 4th bit being either 1 or 0 as was in the input.

Pop. Pop. Pop.

Update:
My reply fell of the edge of the world, so I've reproduced it here: 20 characters: 
sub X{
($_=pop)&pop|~$_&pop
}

[download]

In reply to Re^2: 'Holy in one' another Golf Game! by tadman
in thread 'Holy in one' another Golf Game! by mitd

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