They work somewhat like pragmas in that they tend to affect the compilation of your program, and thus will usually work well only when used within a use, or no. Most of these are lexically scoped, so an inner BLOCK may countermand any of these by saying:
no integer;
no strict 'refs';which lasts until the end of that BLOCK.
Unlike the pragmas that effect the $^H hints variable, the use vars and use subs declarations are not BLOCK-scoped. They allow you to predeclare a variables or subroutines within a particular file rather than just a block. Such declarations are effective for the entire file for which they were declared. You cannot rescind them with no vars or no subs.
The following pragmas are defined (and have their own documentation).
"use autouse MODULE => qw(sub1 sub2 sub3)" - Defers require MODULE until someone calls one of the specified subroutines (which must be exported by MODULE). This pragma should be used with caution, and only when necessary.
blib - manipulate @INC at compile time to use MakeMaker's uninstalled version of a package
diagnostics - force verbose warning diagnostics
integer - compute arithmetic in integer instead of double
less - request less of something from the compiler
lib - manipulate @INC at compile time
locale - use or ignore current locale for builtin operations (see perllocale)
ops - restrict named opcodes when compiling or running Perl code
overload - overload basic Perl operations
re - alter behaviour of regular expressions
sigtrap - enable simple signal handling
strict - restrict unsafe constructs
subs - predeclare sub names
vmsish - adopt certain VMS-specific behaviors
vars - predeclare global variable names
Standard, bundled modules are all expected to behave in a well-defined manner with respect to namespace pollution because they use the Exporter module. See their own documentation for details.
AnyDBM_File - provide framework for multiple DBMs
AutoLoader - load functions only on demand
AutoSplit - split a package for autoloading
Benchmark - benchmark running times of code
CPAN - interface to Comprehensive Perl Archive Network
CPAN::FirstTime - create a CPAN configuration file
CPAN::Nox - run CPAN while avoiding compiled extensions
Carp - warn of errors (from perspective of caller)
Class::Struct - declare struct-like datatypes
Config - access Perl configuration information
Cwd - get pathname of current working directory
DB_File - access to Berkeley DB
Devel::SelfStubber - generate stubs for a SelfLoading module
DirHandle - supply object methods for directory handles
DynaLoader - dynamically load C libraries into Perl code
English - use nice English (or awk) names for ugly punctuation variables
Env - import environment variables
Exporter - implements default import method for modules
ExtUtils::Embed - utilities for embedding Perl in C/C++ applications
ExtUtils::Install - install files from here to there
ExtUtils::Liblist - determine libraries to use and how to use them
ExtUtils::MM_OS2 - methods to override Unix behaviour in ExtUtils::MakeMaker
ExtUtils::MM_Unix - methods used by ExtUtils::MakeMaker
ExtUtils::MM_VMS - methods to override Unix behaviour in ExtUtils::MakeMaker
ExtUtils::MakeMaker - create an extension Makefile
ExtUtils::Manifest - utilities to write and check a MANIFEST file
ExtUtils::Mkbootstrap - make a bootstrap file for use by DynaLoader
ExtUtils::Mksymlists - write linker options files for dynamic extension
ExtUtils::testlib - add blib/* directories to @INC
Fatal - make errors in builtins or Perl functions fatal
Fcntl - load the C Fcntl.h defines
File::Basename - split a pathname into pieces
File::CheckTree - run many filetest checks on a tree
File::Compare - compare files or filehandles
File::Copy - copy files or filehandles
File::Find - traverse a file tree
File::Path - create or remove a series of directories
File::Spec - portably perform operations on file names
File::Spec::Functions - function call interface to File::Spec module
File::stat - by-name interface to Perl's builtin stat() functions
FileCache - keep more files open than the system permits
FileHandle - supply object methods for filehandles
FindBin - locate directory of original Perl script
GDBM_File - access to the gdbm library
Getopt::Long - extended processing of command line options
Getopt::Std - process single-character switches with switch clustering
I18N::Collate - compare 8-bit scalar data according to the current locale
IO - load various IO modules
IO::File - supply object methods for filehandles
IO::Handle - supply object methods for I/O handles
IO::Pipe - supply object methods for pipes
IO::Seekable - supply seek based methods for I/O objects
IO::Select - OO interface to the select system call
IO::Socket - object interface to socket communications
IPC::Open2 - open a process for both reading and writing
IPC::Open3 - open a process for reading, writing, and error handling
Math::BigFloat - arbitrary length float math package
Math::BigInt - arbitrary size integer math package
Math::Complex - complex numbers and associated mathematical functions
Math::Trig - simple interface to parts of Math::Complex for those who need trigonometric functions only for real numbers
NDBM_File - tied access to ndbm files
Net::Ping - Hello, anybody home?
Net::hostent - by-name interface to Perl's builtin gethost*() functions
Net::netent - by-name interface to Perl's builtin getnet*() functions
Net::protoent - by-name interface to Perl's builtin getproto*() functions
Net::servent - by-name interface to Perl's builtin getserv*() functions
Opcode - disable named opcodes when compiling or running Perl code
Pod::Text - convert POD data to formatted ASCII text
POSIX - interface to IEEE Standard 1003.1
SDBM_File - tied access to sdbm files
Safe - compile and execute code in restricted compartments
Search::Dict - search for key in dictionary file
SelectSaver - save and restore selected file handle
SelfLoader - load functions only on demand
Shell - run shell commands transparently within Perl
Socket - load the C socket.h defines and structure manipulators
Symbol - manipulate Perl symbols and their names
Sys::Hostname - try every conceivable way to get hostname
Sys::Syslog - interface to the Unix syslog(3) calls
Term::Cap - termcap interface
Term::Complete - word completion module
Term::ReadLine - interface to various readline packages
Test::Harness - run Perl standard test scripts with statistics
Text::Abbrev - create an abbreviation table from a list
Text::ParseWords - parse text into an array of tokens
Text::Soundex - implementation of the Soundex Algorithm as described by Knuth
Text::Tabs - expand and unexpand tabs per the Unix expand(1) and unexpand(1)
Text::Wrap - line wrapping to form simple paragraphs
Tie::Hash - base class definitions for tied hashes
Tie::RefHash - base class definitions for tied hashes with references as keys
Tie::Scalar - base class definitions for tied scalars
Tie::SubstrHash - fixed-table-size, fixed-key-length hashing
Time::Local - efficiently compute time from local and GMT time
Time::gmtime - by-name interface to Perl's builtin gmtime() function
Time::localtime - by-name interface to Perl's builtin localtime() function
Time::tm - internal object used by Time::gmtime and Time::localtime
UNIVERSAL - base class for ALL classes (blessed references)
User::grent - by-name interface to Perl's builtin getgr*() functions
User::pwent - by-name interface to Perl's builtin getpw*() functions