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

use strict;
use warnings;
...
    '3' => $m++
);
print Dumper(\%hash2);

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