Hidden features of Perl? -

what useful esoteric language features in perl you've been able employ useful work?

guidelines:

• try limit answers perl core , not cpan
• please give example , short description

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hidden features found in other languages' hidden features:

(these corion's answer)

• c
  • duff's device
  • portability , standardness
• c#
  • quotes whitespace delimited lists , strings
  • aliasable namespaces
• java
  • static initalizers
• javascript
  • functions first class citizens
  • block scope , closure
  • calling methods , accessors indirectly through variable
• ruby
  • defining methods through code
• php
  • pervasive online documentation
  • magic methods
  • symbolic references
• python
  • one line value swapping
  • ability replace core functions own functionality

other hidden features:

operators:

• the bool quasi-operator
• the flip-flop operator
  • also used list construction
• the ++ , unary - operators work on strings
• the repetition operator
• the spaceship operator
• the || operator (and // operator) select set of choices
• the diamond operator
• special cases of m// operator
• the tilde-tilde "operator"

quoting constructs:

• the qw operator
• letters can used quote delimiters in q{}-like constructs
• quoting mechanisms

syntax , names:

• there can space after sigil
• you can give subs numeric names symbolic references
• legal trailing commas
• grouped integer literals
• hash slices
• populating keys of hash array

modules, pragmas, , command-line options:

• use strict , use warnings
• taint checking
• esoteric use of -n , -p
• cpan
• overload::constant
• io::handle module
• safe compartments
• attributes

variables:

• autovivification
• the $[ variable
• tie
• dynamic scoping
• variable swapping single statement

loops , flow control:

• magic goto
• for on single variable
• continue clause
• desperation mode

regular expressions:

• the \g anchor
• (?{}) , '(??{})` in regexes

other features:

• the debugger
• special code blocks such begin, check, , end
• the data block
• new block operations
• source filters
• signal hooks
• map (twice)
• wrapping built-in functions
• the eof function
• the dbmopen function
• turning warnings errors

other tricks, , meta-answers:

• cat files, decompressing gzips if needed
• perl tips

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see also:

• hidden features of c
• hidden features of c#
• hidden features of c++
• hidden features of java
• hidden features of javascript
• hidden features of ruby
• hidden features of php
• hidden features of python
• hidden features of clojure

the flip-flop operator useful skipping first iteration when looping through records (usually lines) returned file handle, without using flag variable:

while(<$fh>) {   next if 1..1; # skip first record   ... } 

run perldoc perlop , search "flip-flop" more information , examples.