I have a possibly bad habit to compact boring parts of my code like this:

if( exists($hash->{akey}) && defined($m=$hash->{akey}) && ($m==1) ){ $
+y = $m; ... }
# don't trust $m here
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But in this case it has unexpected results. This is the 1st part where a hash is constructed based on whether a key in another hash exists:

use strict;
use warnings;

use Data::Dumper;

my %tests = (
    'a' => 10,
    'b' => 20,
);
my $m;
# this seems to assign $m once and never bother to check again
my %hash = (
    'b' => exists($tests{'b'}) && defined($m=$tests{'b'}) ? $m : 0,
    'a' => exists($tests{'b'}) && defined($m=$tests{'a'}) ? $m : 0,
);
print Dumper(\%hash);
# this works as expected
$m = 10;
my %hash2 = (
    '1' => $m++,
    '2' => $m++,
    '3' => $m++
);
print Dumper(\%hash2);
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$VAR1 = {
          'a' => 10,
          'b' => 10
        };
$VAR1 = {
          '1' => 10,
          '2' => 11,
          '3' => 12
        };
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Does anyone have an explanation? And is my habit bad?