You should never use a memory-address as a “key.”

Have you heard of inside-out objects? At the base, they do exactly what just proclaimed shouldn't be done.

{
   my %objs;

   sub new {
      my $obj = { ... };
      my $key = 0+$obj;
      $data{$key} = $rec;
      return $key;
   }
}
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(Blessing omitted for clarity.)

I don't know to what side-effect you are referring. That numification of a reference returns its address? That's safe.