One thing to consider when evaluating "why" Perl is (generally) slower than a compiled language, like C, is that tokenization and parsing happens once for compiled programs, but every time you run a Perl script.

As for speed in general, I imagine it depends on what you do with the language. My only data point is on some fairly brute-force statistics (regression analysis): I ran pretty much the same program in C and in Perl, and the C program ran about 200 times faster. I suspect that Perl is much closer to C in areas like string manipulation, where Perl has very heavily optimized (C) primitives. Keep in mind that, since Perl is implemented in C, optimizations being equal C will always be (even just a little bit) faster than Perl, since Perl scripts will be running code written in C, plus the tokenizer and parser.

Language speed depends heavily on how good an optimizer the compiler or interpreter has, and how well it can take advantage of the hardware. That said, program speed is often dependent on the algorithms you use, not the language; if speed is important to you, choose a language that makes writing quick algorithms easy before worrying about this kind of issue.