programming is about algorithms.
fortunately, perl does a lot of work for you by offering data structures like hashes for free. but you should still have a clue what takes time and what not. you should know about algorithms in general and also a bit about the performance of the language you program in.
cpu cycles can matter faster than you think. if you program an application framework that uses modules, you can program for maximum maintainability. if you program a module which gets executed by a framework very often you might want to benchmark a bit. think about DBI. imagine it was written in pureperl and without any care for performance - oh my god, database interaction would be soo slow in perl. if no module author would care about speed, perl itself would be slow because cpan is part of the language somehow. i agree that a very small difference doesn't usually matter because it might be just a platform/version issue that changes in the next version, but to know how to benchmark and to get a feeling about efficiency does not hurt.