That's an ironic statement, since Perl came first and was absolutely dominant in the domains which PHP, Python, Ruby, and Javascript now occupy. When they were young, it would have been reasonable to say that space seems tight for another scripting language because we had Perl.

Languages like Lua have something special. What's special about Perl?

What's special about Lua? That's a four-word question that could require a huge response. Lua's a nice little embeddable scripting language, but that's a niche that Perl has occupied too. It's got fantastic support for both being embedded (callable from other languages) and embedding (able to call other languages). You might say that Lua's better because it's smaller and cleaner and simpler, but that's only true until you run into Lua not being powerful enough for something you want to do. If you used Perl as your embedded scripting language instead, you probably wouldn't have that problem. (Other problems, maybe, but not a lack of power.)