If it's intended interpolation, then you have probably made a typo in the name of the variable. The warning is saying that you are trying to interpolate an array whose name is unknown.

If you don't intend to interpolate it (e-mail addresses are notorious for triggering this), just add backslash (\) before the @. That tells the tokeniser that it is a literal @, and not the opening sigil of an array to be interpolated.

If you don't care either way, write:

  use warnings;
  no warnings 'ambiguous';
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and then the message will never appear.