Perl is not strictly an interpreted language; rather, it is compiled into an intermediate bytecode which is then interpreted. This initial compilation, along with loading the Perl interpreter into memory, takes a moment or two, and may lead users to think that Perl itself is a slow language.
Perl's interpreter is written in C and has been optimised and tweaked for more than a decade. For most purposes, Perl is about as fast as anything else you might use - at least for most everyday purposes.
If you are using Perl for heavy mathematics, the Perl Data Language (PDL) provides added features and improves performance.
If you are using Perl for CGI programming, you can use a server add-on such as Apache's mod_perl to keep the Perl interpreter in memory and save on the startup time overhead.