Use as much pure-perl as you can, and avoid low-level activities at all cost. Do not anticipate given/when using a Filter, because filters only reduce the chance of having a good conversion.

Try downloading the Perl 1.0 sourcecode, that was released in 1987. You'll be amazed to see how many tests are succesful if you run them with Perl 5.6.1. That is mostly because the code is written simple and clear. The tests that don't succeed all have to do with things you want to avoid anyway, like weird variable names ($  (dollar space)), goto LABEL and some minor language incompatibilities that I don't understand (it seems that Perl 1.0 subs return undef if their last statement is conditional and never executed).

Someone else has already mentioned things you should avoid to ensure easier conversion. Anyhow, write clear pure-perl code, in a good style - make sure that you don't do things that "happen to work". If you have some spare time, write tests.

Oh and use strict, especially when writing modules.

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