Sorry, I should have provided more context. I was looking at an earlier version which I had changed to

{
  my %rtoa = ( M=>1000, D=>500, C=>100, L=>50, X=>10, V=>5, I=>1 );
  sub roman_to_dec {
    reduce { $a+$b-$a%$b*2 } @rtoa{ split//, uc(shift) }
  }
}
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