What are some really useful but esoteric language features in Perl that you've actually been able to employ to do useful work?

Guidelines:

• Try to limit answers to the Perl core and not CPAN
• Please give an example and a short description

Hidden Features also found in other languages' Hidden Features:

(These are all from Corion's answer)

• C#
  • Duff's Device
  • Portability and Standardness
  • Quotes for whitespace delimited lists and strings
  • Aliasable namespaces
• Java
  • Static Initalizers
• JavaScript
  • Functions are First Class citizens
  • Block scope and closure
  • Calling methods and accessors indirectly through a variable
• Ruby
  • Defining methods through code
• PHP
  • Pervasive online documentation
  • Magic methods
  • Symbolic references
• Python
  • One line value swapping
  • Ability to replace even core functions with your own functionality

Other Hidden Features:

Operators:

• The bool quasi-operator
• The flip-flop operator
  • Also used for list construction
• The ++ and unary - operators work on strings
• The repetition operator
• The spaceship operator
• The || operator (and // operator) to select from a set of choices
• The diamond operator
• Special cases of the m// operator
• The tilde-tilde "operator"

Quoting constructs:

• The qw operator
• Letters can be used as quote delimiters in q{}-like constructs
• Quoting mechanisms

Syntax and Names:

• There can be a space after a sigil
• You can give subs numeric names with symbolic references
• Legal trailing commas
• Grouped Integer Literals
• hash slices
• Populating keys of a hash from an array

Modules, Pragmas, and command-line options:

• use strict and use warnings
• Taint checking
• Esoteric use of -n and -p
• CPAN
• overload::constant
• IO::Handle module
• Safe compartments
• Attributes

Variables:

• Autovivification
• The $[ variable
• tie
• Dynamic Scoping
• Variable swapping with a single statement

Loops and flow control:

• Magic goto
• for on a single variable
• continue clause
• Desperation mode

Regular expressions:

• The \G anchor
• (?{}) and '(??{})` in regexes

Other features:

• The debugger
• Special code blocks such as BEGIN, CHECK, and 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 into errors

Other tricks, and meta-answers:

• cat files, decompressing gzips if needed
• Perl Tips

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