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

##</code><code>##

use strict;
use warnings;
use Data::Dump qw/pp dd/;


my %tests = (
    'a' => 10,
    'b' => 20,
);

my $m;

# this seems to assign $m once and never bother to check again

my %hash;

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

pp \%hash;

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

pp \%hash;

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

pp \%hash;

##</code><code>##

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