So I am currently asking myself: Is the advantage of the comfortable Visual Studio, the tools, the framework, ... worth chooing ASP.Net over PHP or Perl? Are the higher licence and hosting costs worth the ease of coding?
I think this is really going to be a question of personal preference. If it were me, I would consider the Windows platform and IDE a disadvantage. Anything that's not emacs is junk.
For me, that would narrow it down to something like Perl, Lisp, Haskell, Python, Ruby, or PHP. Since you mentioned PHP and Perl, and I know both, I will speak to those.
PHP is easy to get started with. You drop a file in the directory, and it works. Copy it to the server and it works.
Unfortunately, the language is missing many essential modern features. All variables are global. The OO system is broken. There is no (coherent) Unicode string support. There is no culture of writing libraries. There is no culture of automatic testing.
Perl is pretty much the opposite. When you deploy your apps, you will probably need to install a bunch of CPAN modules. Perl programmers LOVE library reuse. So, when you're getting started, you'll need to learn how to install libraries, because you won't get far in Perl without them. When you deploy your application, you'll have to choose between plain HTTP + Reverse Proxy, or mod_perl, or FastCGI. There are a few major web frameworks (though Catalyst is generally recommended as the Standard). There are even different object systems (but again, Moose is what you want). You'll spend a bit more time configuring your system, and a bit more time choosing libraries with Perl.
But, the idea is that Perl will give you much more choice than any other language. Things will work exactly as you want them to. The community is super-active (there is an independently organized Perl conference at least once every month, and the irc channels and mailing lists are always busy and always helpful). Every module ships with a test suite, to ensure that the library is working correctly on your exact machine and configuration. (No guessing!)
So basically, if you are willing to trade a higher learning curve for better libraries and community, use Perl. If you don't care much about what you are doing, and just want to be done, pick PHP.