Personally, I would avoid the first choice whenever possible. What about when your code looks like this:

--$option > ($i - 1) && $a = shift;
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Not the most complex logic in the world, but IMHO you've already lost a lot of readability. As for the other two, I like the b() if expr; when there's only one expression to do, and the traditional if block any other time. Also, I personally find Stroustoup-style indenting/bracketing the most readable, ie.

if ( expr )
{
    b() ;
}
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I'm aware that this bracketing isn't the convention in Perl (if anything is a convention; TMTOWTDI), but it's my personal preference. What do others think?