Alternative - yes, better - ???

Why use a map - "slices forever !!"

{
  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)}
  }
}
[download]

Or double your fun with a double-split.

sub roman_to_dec { reduce { $a+$b-$a%$b*2 }
  @{{split/(\d+)/, 'M1000D500C100L50X10V5I1'}}{split//, uc pop}
}
[download]

Or back to a map...

sub roman_to_dec { reduce { $a+$b-$a%$b*2 }
  map 'M1000D500C100L50X10V5I1' =~ /$_(\d+)/i, split//, pop
}
[download]

Well, that was fun - thanks