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  m[i,i] = 0
  m[i,j] =  min  { m[i,k] + m[k+1,j] + p[i-1]*p[k]*p[j] }
           i<=k<j

use Memoize;
use List::Util 'min';
...
    return min map { m($i,$_) + m($_+1,$j) + $P[$i-1]*$P[$_]*$P[$j] }
               $i .. $j-1;
}