I never realized until recently how hard something like this is. People do not want to move. People that hear about your job post usually have some reason to have heard about your job post, like they can't program at all and their last employer finally figured it out. Unfortunately, you can't know this until they show up and you ask them a few questions about Perl. (I love how one candidate tried to BS
me on a basic Perl question. "Do you know OO Perl?" "Yes." "What's the first argument to a Perl method?" "$_". "Don't you mean $_[0]?" "They're the same thing." Congratulations, you have clearly never written any Perl.)
I work at a bank, and we need Perl programmers too. We can afford to pay a lot better than booking.com, but we can't find anyone, either. It's very odd. (And it's not just Perl, we had the same problem with Java and C#, and are having the same problem with C++. Might as well rewrite our app in Haskell... at least those languages have a community.)
The other thing that I see as being difficult for booking.com is that people don't want to work with 40+ other people. Between learning "the rules" from the people already there and teaching the new folks how to program, there is never any time to get anything done. (My second "real job" was like this; I was not even a "team lead" and yet all I did was teach other people basic Perl + Database stuff. We got exactly no work done. At least if I was by myself I would have been able to do one person's work.)
In fact, I find the other Perl team in my office to be too big -- and they only have 4 people!