Another, slightly less secure, option is to do /usr/bin/env perl5_6_1 and that will execute the first perl5_6_1 in your $PATH. The reason it's less secure is that it depends on your path, which could be hijacked. It's always best, in production code, to be as explicit as possible.

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We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.