my %from_roman = (
    I => 1,
    V => 5,
    X => 10,
    L => 50,
    C => 100,
    D => 500,
    M => 1000,
);
sub from_roman (_) {
    my ($roman) = @_;
    my $n = 0;
    while ($roman =~ s/(I[VXLCDM]|X[LCDM]|C[DM])//) {
        my ($minus, $plus) = split //, $1;
        $n += $from_roman{$plus} - $from_roman{$minus};
    }
    $n += $from_roman{$_} for split //, $roman;
    $n
}