$ cat t.pl
use strict;
use warnings;

use Data::Dumper;

my %tests = (
    'a' => 10,
    'b' => 20,
    'd' => 40,
);
my ($m, $n);
# this seems to assign $m once and never bother to check again
my %hash = (
    'd' => exists($tests{'d'}) && defined($n=$tests{'d'}) ? $n : 0,
    'b' => exists($tests{'b'}) && defined($m=$tests{'b'}) ? $m : 0,
    'c' => exists($tests{'c'}) && defined($m=$tests{'c'}) ? $m : 0,
    'a' => exists($tests{'a'}) && defined($m=$tests{'a'}) ? $m : 0,
);
print Dumper(\%hash);

$ perl t.pl
$VAR1 = {
          'c' => 0,
          'b' => 10,
          'd' => 40,
          'a' => 10
        };