(1) Don't worry about performance unless there's an obvious performance problem. Especially don't worry about the performance of little features. This is called "premature optimization."
(2) When you DO find a serious performance problem, attack the worst performance areas first. How do you know what's the worst? You Devel::Profile the code as a whole and Benchmark alternatives. Profiling and Benchmarking is the only sane way of discovering what portion of your program is taking the most time. Update: clarified myself reflecting L~R's corrections.
(3) The specific feature you mention should have almost no effect on execution time, since the function name is entered into both symbol tables. The only difference I would guess would be how long it took the parser to parse the extra four tokens.
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