With Perl 5.12 comes a working test example already using the pluggable feature: http://search.cpan.org/dist/perl/ext/XS-APItest-KeywordRPN/K...

Here is the code example from the docs:

    use XS::APItest::KeywordRPN qw(rpn calcrpn);

    $triangle = rpn($n $n 1 + * 2 /);

    calcrpn $triangle { $n $n 1 + * 2 / }