That's because decorate has an even higher precedence (on the left of any operator except commas, and unless parenthesis are involved) so what happens is actually: 

(decorate("foo"), decorate("bar").'!')
((return $decorated_foo), (return $decorated_bar.'!'))
((return $decorated_foo), (return $decorated_bar_with_exclamation_mark
+))
($decorated_foo, (return $decorated_bar_with_exclamation_mark))
($decorated_foo, $decorated_bar_with_exclamation_mark)

[download]
(This is of course, not actual code, but just a representation)
So first the calls to decorate are resolved, then the concatenation, and at last the values are added to the list. But the concatenation does not happen last.

Edit: "removed" a bit about precedence being higher on the left of some operators, because it's late, and I'm not sure about what I'm saying.

In reply to Re^3: Why does the first $c evaluate to the incremented value in [$c, $c += $_] ? by Eily
in thread Why does the first $c evaluate to the incremented value in [$c, $c += $_] ? by smls

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