Puts "#{expression}" braces always needed?

The pickaxe book says (on page 51) that if the expression is a global, class
or instance variable then the braces aren’t required. Omitting them doesn’t
seem to work for me. Is the book wrong?

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The pickaxe book says (on page 51) that if the expression is a global, class
or instance variable then the braces aren't required. Omitting them doesn't
seem to work for me. Is the book wrong?

it seems to work for me

pigeon% ruby -e '$a = @@a = @a = 12; p "#$a #@@a #@a"'
"12 12 12"
pigeon%

Can you give an example ?

Guy Decoux