I actually had this very discussion with stvn this morning, but with respect to Ruby's signatures. To learn Ruby, I've been porting a Tree module that I just wrote in Perl to Ruby. One of the things I would like to be able to do is:

def add_child( Hash options is optional, Tree *children )
    # Subroutine body here
end
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Since I cannot do that, I have to do something like:

def add_child( *children )
    options = children[0].is_a( 'Hash' ) ? children.shift : Hash.new;
    if children.any? { |node| !node.is_a( 'Tree' ) }:
        raise ArgumentError "Non-Tree argument found in add_child()"
    end

    # Resume regular processing here
end
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I'd much rather have the subroutine signature do that for me. Perl6 will have this and I hope Ruby2.0 will, but we'll have to see.