I think you're forgetting that my produces a lexical variable that isn't attached to a package namespace. Since you're creating them inside a string eval, they go out of scope as soon as it ends, leaving you with the same variables you had when you entered that string eval (that is, none).

For a quick strict-compliant fix, change all of your mys to ours—alternatively, use vars qw($fee $fi $fo @fum); and dropping the my entirely would have a similar effect.