Perhaps your example is contrived, but defining a manifest constant named FIRST for 1 seems like magic number avoidance run amok. Regardless,

  use constant FIRST => 1;
  use constant SECOND => 2;
  use constant FIFTH => 5;

  %nums = (
      &FIRST => 'Ein',
      &SECOND => 'Zwei',
      &FIFTH => 'Funf'
  );
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works, though to use the same constants as keys, you need to say $nums{&FIRST} or $nums{+FIRST}