In the sub that works you use

$self->{}
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explicitly dereferencing the $self reference. In the sub that doesn't work you use

$self{}
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which implies that $self is a hash itself and not a reference -- thus making your use strict; complain about an undefined global hash called %self.

As a side note I'd suggest dropping the double quotes in your hash keys -- they're not necessary and they force interpolatin where none is necessary (even if you needed interpolation you wouldn't need quotes there).