It's basically the same reason why:

    m / foo  # /
        bar /x;
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fails to do what you expect it to do. Perl will first try to find the entire 'token' before interpreting it. So, in your case, Perl first tries to find the entire string (that is, it has to determine the end of the string), before it pays attention to whether anything needs to be interpolated.

Abigail