See this http://www.perl.com/doc/FMTEYEWTK/comp-vs-interp.html
Loosely speaking, the real perl executable eg /usr/bin/perl reads your Perl code and sort-of-compiles it into an 'executable environment' in RAM, which it then runs.
Hence its much faster than a true interpreted lang eg bash, but not quite as fast as say 'C'.
My rule of thumb is Perl is ~85% as as fast as C, all other things being equal.
If you really want to get into the details, ask over at www.perlmonks.org;
where the Perl gurus hang out.