Learning the basics of new language is easy. Or as least, easier than doing things with an inadequate language. And because you want "quick and dirty", you don't need to know all the subtilities of you language of choice before you start coding.
You are talking about web dev, mobile dev, desktop GUIs and text file processing. These are very different use cases and I think that using the same language for all would be silly, especially for one-shot apps. I couldn't find something that beats Perl for text-file processing, PHP is really good for web pages, if you make GUIs for windows, don't forget about VB. I don't know much about the mobile world so I can't help you there.
As for Python, it is certainly a very nice language but I wouldn't recommend it : too "clean". With one-shot dev, you can throw away readability, modularity, maintainability, etc... so you'll probably want a very permissive (ugly) language like Perl. Of course, it also depends on the available libraries. Both Perl and Python have a large catalog but if the ones you need are only available in one language, then use it.