In this case I'd rather replace the sort block with a function, it's match easier to set breakpoints and watch expressions then.

  DB<1> sub by_num { return $a <=> $b }

  DB<2> x [sort by_num 1,11,2,12]
0  ARRAY(0x334b888)
   0  1
   1  2
   2  11
   3  12
  DB<3>
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update

added return to make mechanism obvious.