It's generally best to assume that the software will need to be maintained and that you should code with that in mind.

What's more, even a oneliner tends to break in a hard to find way if no strict and warnings are enabled. My experience is that especially in "duct tape mode" I tend to shuffle around scripts, paths and files.

Taking the effort of enabling warnings and declaring my variables saves me a lot of debugging effort, even for scripts that solve one ad-hoc problem in an ad-hoc way.

Many times such a "proof of concept" later became part of a more serious project. Then it pays to have a more robust start.

(on a side note, I'm currently really interested in the eXtreme Programming method, where you start to design testcases rather than jumping at solving a problem by hacking away).

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Cheers, Joe