It seems only 5 years ago people pushed me to learn Perl over Python. I saw many tutorial and codes done in the language. Now, however, people hate it. They say it looks ugly, and preforms horribly. I acknowledge it fell behind other dynamic languages in web app development, but it is not a bad language. CPAN is the *most* amazing network of libraries I have ever seen. I've seen codes to monitor ARP, or even water plants periodically in Perl. The language seems to be extremely powerful and flexible. Why did it fall out so quickly?
This is how technology evolves. Perl played a massive part in evolving a different programming paradigm but every language soon or later starts to lose popularity to new languages. Some last longer and are able to keep up with the fast changes and some aren't.
As a programmer you should ALWAYS be open to change, specially when it comes to programming languages. Remember the language your program in is just a tool that you use to reach your goal. New tools will come and go and you have to be willing to adapt and use them when they suit your needs.
Though with all that said Perl is far from being dead it is still very actively used in many projects. It might not be a huge mainstream language but it is still in active use.
Though as to why Perl is not mainstream there is probably a lot of contributing factors one of them probably being the very long development times of the Perl releases (Perl 6?). Though I really haven't worked with Perl enough to truly comment on why it is in decline so I can only really guess as to why.
Perl is incredibly ugly, but it's always been that way. Sysadmins like it because it's on literally every *nix distro by default, it's got some amazing regex support, and it's just a solid scripting language. You can get a lot done with it, but yeah I personally haven't seen it used much on anything large.
Ditto with the ugliness issue. Most commonly given excuse for the demise of Perl that Ive heard is that its not standardized enough in terms of formatting, which is annoying for companies maintaining code with different developers.
Generally speaking though, I think Python just built up too much momentum, and snowballed. Once Python usage rose above a certain point, lots of people started creating libraries for it, which led to more users, which led to more libraries, which led to more users...
You get the idea.
I still think its a decent language though. I plan on learning it eventually just for the heck of it, but I still have quite a few things to accomplish before that.
I've heard is that it is a primary language used in bioinformatics. Mostly because it is so geared towards working with strings which is a big part of what bioinformatics do, and partly because there is so much existing pearl bioinformatics code to work with.