TL;dr

but that's really what you want or just a C&P disaster? ;)

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

update

OK now I looked closer, I think you get bitten by passing the same lvalue $m twice in the same statement.

update

yep, seems I'm right, if you force Perl to pass an immutable copy instead of the var it works like expected

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;
[download]

{ a => 10, b => 10 }
{ a => 10, b => 20 }
{ a => 10, b => 20 }
[download]

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