You could further simpify (and speed up) the code:

for my $letter ( split // => $phrase ) { 
         $sum += $numeral{$letter} || 0; 
 }
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to just

use List::Util qw(sum); # in the CPAN

$sum = sum map $numeral{$_}, $phrase =~ /([IVXCLMD])/g;
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Remember list context m//g. Once you get your head around it, you can do many things for which split seemed awkward.

-- Randal L. Schwartz, Perl hacker