$ perl -MO=Deparse -e '$hash{__PACKAGE__} = 1'

$hash{'__PACKAGE__'} = 1;
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__PACKAGE__ gets treated as a string, as does any bareword (keyword or not) that you use to index a hash (think $foo{shift}). The easiest fix is adding a plus sign to make it an expression instead of a string.

$ perl -MO=Deparse -e '$hash{+__PACKAGE__} = 1'

$hash{'main'} = 1;
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