IMO

I am occasionally asked "How do you pick which language to use for a partcular project?" Usually the question concerns various scripting languages, particularly PHP, Perl and Python. The sad fact is that there are no cut-and-dried rules; it depends greatly on the design requirements of the project in question. In general though, I find myself following the below general pattern:

Perl:

For short scripts or tools, Perl is the way to go. I hardly every use 'sh' anymore for basic admin things either. It also excels for basic proof-of-concept things. Anytime you can do stuff like $sp{'foo'}[$idex]->{'bar'} = $spamalot; and have everything pop into existence automagically, life is good.

PHP:

For any web-application, I tend to steer towards PHP. A full and robust suite of functions and capabilities tuned with web-apps in mind.

Python:

For any medium to large stand-alone application, Python is the best choice. Straddles the high-level/low-level language fence very well, just right as far as the amount of "basic" functionality provided, and an extensive array of library modules.

Of course, there is overlap... I find myself using mod_python more and more, eating a little bit into the PHP space... but then again, I haven't made the PHP5 jump yet, which may just turn that around. Also, if the project requires a lot of textual abstraction and manipulation, then it just screams out for Perl, no matter what the size or implementation.