my %hash = (
    'b' => exists($tests{'b'}) && defined($m=$tests{'b'}) ? $m : 0,
    'a' => exists($tests{'a'}) && defined($m=$tests{'a'}) ? $m : 0,
);

##</code><code>##

my %hash = (
   a => $tests{a} // 0,
   b => $tests{b} // 0,
);