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

def roman_to_dec(r, __rtoa = dict(M=1000, D=500, C=100, L=50, X=10, V=
+5, I=1) ):
  return reduce( lambda t,n: t+n-t%n*2, (__rtoa[c] for c in r.upper())
+ )