eval­("\$x­$_\{­EOS}=­0");
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With $_ I get this warning:

    Use of uninitialized value $_{"EOS"} in concatenation (.) or strin
+g at -e line 1, <> line X.
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Works for me. Based on the error, it looks like you failed to include the \ between $_ and {EOS}.

Considering the affectation {­EOS}=0, I wouldn't call it a bug as I didn't mean to get the string "EOS" in my tree.

If you didn't want that, then you should replace your {EOS} with ={} so the intent is clear instead of looking very much like a bug.

Even if it isn't a bug compared to your intention, it is still a bug if you are trying to make a trie. With your implementation you can't tell that both "hill" and "hilly" are stored in one structure.