Problem is not with your $rules nor $rules2, it's with your driver=> line.

Change your code to this and your code with work -

    my %has =
    (
     # japhy was right, use [ ] to return reference to array
     driver => [grep { $p{rdbms} eq $_ } Alzabo::Driver->available],
     rules  => (scalar grep { $p{rdbms} eq $_ } Alzabo::RDBMSRules->av
+ailable)
     ? 'yes' : 'no',
     rules2 => (scalar grep { $p{rdbms} eq $_ } Alzabo::RDBMSRules->av
+ailable)
    );
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