From perldata (emphasis mine):

The special literals __FILE__, __LINE__, and __PACKAGE__ represent the current filename, line number, and package name at that point in your program. They may be used only as separate tokens; they will not be interpolated into strings. If there is no current package (due to an empty package; directive), __PACKAGE__ is the undefined value.

The original code gives:

$VAR1 = {
          '__PACKAGE__' => {
                             '__FILE__' => {
                                             '__LINE__' => 'Mu Mu'
                                           }
                           }
        };
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But the "cruciform operator" can be used to interpolate tokens:

$hash{+__PACKAGE__}{__FILE__}{__LINE__} = "Mu Mu";
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$VAR1 = {
          'Illumination' => {
                              '__FILE__' => {
                                              '__LINE__' => 'Mu Mu'
                                            }
                            }
        };
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