$m puts the $m itself on the stack (not a copy), so you effectively end up assigning 'b', $m, 'a', $m (where $m is 10).

$m++ puts the old value of $m on the stack, which must necessarily be a new variable, so you end up assigning '1', $anon1, '2', $anon2, '3', $anon3 (where $anon1 is 10, $anon2 is 11 and $anon3 is 12).

---

defined is only true for keys that exists, so exists($h{x}) && defined($h{x}) can be written as defined($h{x}).

defined($x) ? $x : 0 is equivalent to $x // 0 (except that $x is only evaluated once).

So,

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

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