if( exists($hash->{akey}) && defined($m=$hash->{akey}) && ($m==1) ){ $y = $m; ... }
# don't trust $m here

##</code><code>##

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);

##</code><code>##

$VAR1 = {
          'a' => 10,
          'b' => 10
        };
$VAR1 = {
          '1' => 10,
          '2' => 11,
          '3' => 12
        };