I wrote a Perl 6 version (transcoded from the Perl 5 version), which uses the following improvements:

• Instead of another scope for %rtoa, I used a state variable (also available in perl-5.10)
• I wrote the "pipeline" as a set of method calls, which means they are read in the order that they are executed
• Use of a formal parameter
• say instead of print (also available in 5.10)
• (Updated) use hyphens instead of underscores as word separator in the sub name. It's a matter of taste, but I like it better that way.

Here's the code, which works with today's Rakudo:

sub roman-to-dec($x) {
    state %rtoa = M=>1000, D=>500, C=>100, L=>50, X=>10, V=>5, I=>1;
    $x.uc.split('').map({ %rtoa{$_} }).reduce: { $^a+$^b-$a%$b*2 }
}

my @testdata = <XLII LXIX mi>;
for @testdata -> $r {
    say "$r: ", roman-to-dec($r);
}
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