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    'b' => exists($tests{'b'}) && defined($m=$tests{'b'}) ? $m : 0,
    'a' => exists($tests{'b'}) && defined($m=$tests{'a'}) ? $m : 0,
                        ^^^^^

use strict;
use warnings;
...
  );

pp \%hash;

{ a => 10, b => 10 }
{ a => 10, b => 20 }
{ a => 10, b => 20 }