3.2.185 use

use Module LIST
use Module

The use declaration 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 the following:

BEGIN { require Module; import Module LIST; }

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 built-in - it's just an ordinary static method call into the package named by Module 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 Chapter 5 for more information.

If you don't want your namespace altered, explicitly supply an empty list:

use Module ();

That is exactly equivalent to the following:

BEGIN { require Module; }

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

use integer;
use diagnostics;
use sigtrap qw(SEGV BUS);
use strict  qw(subs vars refs);

These pseudomodules typically import semantics into the current block scope, unlike ordinary modules, which import symbols into the current package. (The latter are effective through the end of the file.)

There's a corresponding declaration, no, that "unimports" any meanings originally imported by use, but that have since become, er, unimportant:

no integer;
no strict 'refs';

See Chapter 7 for a list of standard modules and pragmas.