The dot's going to be the method call character, replacing the arrow, so it can't be string concatenation. Underscore's the leading candidate for string concat, though there's been some waffling about that lately.

You will be able to use .= still, but it'll not do string concatenation--it's the shorthand for method call and assign. So this:

$foo .= bar;
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is the same as:

$foo = $foo.bar;
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At the moment there's not really a 5->6 translation document, but I expect there'll be one built before perl 6 comes out.