I just released to CPAN a distribution named Acme-Roman. (It's propagating yet, so maybe you can't reach immediately from your favourite CPAN mirror.) As an Acme module, it is a simple experience to allow using roman numerals in Perl programs in a straightforward manner. 
    use Acme::Roman;

    print I + II; # III, as expected

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That was done with a bit of overload, AUTOLOAD and a part which I am not satisfied with.

Some digression first. The idea came to me a long time ago from the solution to a Ruby quiz, namely

Ruby Quiz - Roman Numerals (#22)
http://rubyquiz.com/quiz22.html
(See the hightlighted solution at the Quiz Summary in the same page.) The trick in Ruby was done by defining strategic methods method_missing (which is kind of Perl AUTOLOAD) and const_missing. The Perl solution was quite natural, and I got to the point where 
    use Acme::Roman;

    print I() + II(); # III

[download]
worked fine. But when I left the parentheses, it only barfed at me with 
    $ perl -w -Ilib -MAcme::Roman -e 'my $x = I+II'
    Argument "II" isn't numeric in addition (+) at -e line 1.
    Argument "I" isn't numeric in addition (+) at -e line 1.

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and 
    $ perl -w -Ilib -MAcme::Roman -e 'print I+II'
    Name "main::I" used only once: possible typo at -e line 1.
    print() on unopened filehandle I at -e line 1.

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which revealed the barewords were not resolved with the provided AUTOLOAD. I learned that if I provided an empty protoype it worked. And that's how I coded -- but that kind of sucks: I need to define the prototype of 3999 subs (I .. MMMCMXCIX) to make it work. I think that's too expensive. (Who would like to play with an Acme module which is as heavy as a useful module? ;-)

The Ruby solution is helped by the availability of const_missing method functionality. The equivalent in Perl would be to overload barewords, but overload documentation says:

Barewords are not covered by overloaded string constants.

Does anyone have any suggestion to make this lazier? Or a different way to implement that?

In reply to AUTOLOAD, overload, and Roman numerals for Perl by ferreira

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