undef is invalid value for hash key, since this warning. If you don't want to see it you can check if $attribute defined or not: 

sub get_attribute {
    my ( $self, $attribute ) = @_;
    return defined($attribute) ? $self->{ATTRIBUTES}{$attribute} : und
+ef;
}

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In reply to Re^6: Use of uninitialized value in hash element at by zwon
in thread Use of uninitialized value in hash element at by Anonymous Monk

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