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in reply to declaring lexical variables in shortest scope: performance?

Don't diddle code to make it faster -- find a better algorithm

-- The Elements of Programming Style

Don’t Optimize Code -- Benchmark It

-- from Ten Essential Development Practices by Damian Conway

It's important to be realistic: most people don't care about program performance most of the time

-- The Computer Language Benchmarks Game

The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming

-- Donald Knuth

Don’t pessimize prematurely. All other things being equal, notably code complexity and readability, certain efficient design patterns and coding idioms should just flow naturally from your fingertips and are no harder to write than the pessimized alternatives. This is not premature optimization; it is avoiding gratuitous pessimization.

-- Andrei Alexandrescu and Herb Sutter

Rule 1: Bottlenecks occur in surprising places, so don't try to second guess and put in a speed hack until you have proven that's where the bottleneck is.
Rule 2: Measure. Don't tune for speed until you've measured, and even then don't unless one part of the code overwhelms the rest.
Rule 3. Fancy algorithms are slow when n is small, and n is usually small. Fancy algorithms have big constants. Until you know that n is frequently going to be big, don't get fancy. (Even if n does get big, use Rule 2 first.)
Rule 4. Fancy algorithms are buggier than simple ones, and they're much harder to implement. Use simple algorithms as well as simple data structures.
Rule 5. Data dominates. If you've chosen the right data structures and organized things well, the algorithms will almost always be self-evident. Data structures, not algorithms, are central to programming.
Note: Pike's rules 1 and 2 restate Tony Hoare's "Premature optimization is the root of all evil". Ken Thompson rephrased Pike's rules 3 and 4 as "When in doubt, use brute force". Rules 3 and 4 are instances of KISS. Rule 5 was stated by Fred Brooks in The Mythical Man-Month and is often shortened to "write stupid code that uses smart objects" (see also data structures vs code).

-- Rob Pike

Without good design, good algorithms, and complete understanding of the program's operation, your carefully optimized code will amount to one of mankind's least fruitful creations -- a fast slow program.

-- Michael Abrash

A couple of related general guidelines from On Coding Standards and Code Reviews:

On Interfaces and APIs cautions that library interfaces are very difficult to change once they become widely used - a fundamentally inefficient interface cannot be easily fixed later by optimizing. So it is not "premature optimization" to consider efficiency when designing public library interfaces.

See Also

These experiences convinced me of don't assume measure and especially find a better algorithm!

Perl Performance References

High Performance and Parallel Computing References

Extra Performance/Optimization References Added Later

Bitwise Operations:

Benchmark:

Other:

From BrowserUk (2012-2015):

Two spookily similar nodes posted late 2021 (both requesting XS C code, both by new monks who won't show us their code):

Some old classics:

Other:

On CPAN:

Some external references:

Mathematical:

Memory:

Sorting:

Multi-threading:

Compiler switches/flags:

I/O:

PDL and Array Processing References

See: Re^2: Organizational Culture (Part II): Meta Process (BioPerl/PDL/AI/Embedded/Data Science References)

RPerl References

See Also

Updated: Added Donald Knuth premature optimization quote and Alexandrescu/Sutter premature pessimization quotes and Rob Pike quotes. Mentioned efficiency of interfaces. Added more references.

In Section Seekers of Perl Wisdom