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use Module VERSION
use Module LIST
use Module

Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

BEGIN { require Module; import Module LIST; }

except that Module must be a bareword.

VERSION, which can be specified as a literal of the form v5.6.1, demands that the current version of Perl ($^V or $PERL_VERSION) be at least as recent as that version. (For compatibility with older versions of Perl, a numeric literal will also be interpreted as VERSION.) If the version of the running Perl interpreter is less than VERSION, then an error message is printed and Perl exits immediately without attempting to parse the rest of the file. Compare with "require", which can do a similar check at run time.

use v5.6.1;		# compile time version check
use 5.6.1;		# ditto
use 5.005_03;	# float version allowed for compatibility

This is often useful if you need to check the current Perl version before useing library modules that have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.)

The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import is not a builtin--it's just an ordinary static method call into the Module package to tell the module to import the list of features back into the current package. The module can implement its import method any way it likes, though most modules just choose to derive their import method via inheritance from the Exporter class that is defined in the Exporter module. See Exporter. If no import method can be found then the call is skipped.

If you do not want to call the package's import method (for instance, to stop your namespace from being altered), explicitly supply the empty list:

use Module ();

That is exactly equivalent to

BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument. The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VERSION.

Again, there is a distinction between omitting LIST (import called with no arguments) and an explicit empty LIST () (import not called). Note that there is no comma after VERSION!

Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:

use constant;
use diagnostics;
use integer;
use sigtrap  qw(SEGV BUS);
use strict   qw(subs vars refs);
use subs     qw(afunc blurfl);
use warnings qw(all);

Some of these pseudo-modules import semantics into the current block scope (like strict or integer, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).

There's a corresponding no command that unimports meanings imported by use, i.e., it calls unimport Module LIST instead of import.

no integer;
no strict 'refs';
no warnings;

If no unimport method can be found the call fails with a fatal error.

See perlmodlib for a list of standard modules and pragmas. See perlrun for the -M and -m command-line options to perl that give use functionality from the command-line.