Quoting the referenced documentation:

For array options, a reference to an array is used, e.g.:

  %optctl = ();
  GetOptions (\%optctl, "sizes=i@");
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So you append '@' to the specification. Once I saw that, I searched for @ and found the documentation for this under the "Compatability" section.

Note that I'd use single quotes or back-slash the @ if for no other reason than to make the intent of a literal @ clear.

        - tye (but my friends call me "Tye")