Speed of execution and expressiveness are common factors, and this depends on what you're doing. For instance, regular expressions. If you're doing a program

Speed of execution and expressiveness are common factors, and this depends on what you're doing. For instance, regular expressions. If you're doing a program that is primarily not dealing with text, but occasionally needs a regex, then Python regexes would be fine. If the program does a lot of text processing, it should probably use Perl, due to the performance and built-in syntax. And if the entire program is a simple filter, "sed" is even better than the Perl sledgehammer. The standard library can be a big factor. Not having to download a dependency is a big win, which is why large standard libraries such as Python can often be better than something like a CPAN dependency in Perl. It can be the difference between using C++ or Objective-C, versus C. But more generally, if you need to do X, and you already know how to do it with language Y, you'll need a really good reason to try a different language.