Perl Module Basics

You can use modules without exporting symbols by using require instead of use:

#!/usr/bin/perl

require File::Find;

File::Find::find(sub { print "$File::Find::name\n" if  s > 1_024_000; },  . );

As a result, the reference to find must be prefixed with a full package name and 

written as File::Find::find.

Another difference between the two is that use happens during compile time,

whereas require happens at runtime. Perl runs a program in two phases first, the 

program and all modules used by the program are compiled into an internal byte 

code format. This is known as compile time. Next, the byte code is executed and 

the program actually runs. Perl programs can actually go back and forth between 

runtime and compile time using two mechanisms: BEGIN and eval.

A BEGIN block is a way of getting a bit of runtime during compile time. When 

Perl encounters a BEGIN block, it executes the code found inside the BEGIN block as 

soon as it has been compiled, before any code that comes later in the program. For 

example, this line will print even if there's a compilation error later in the script:

BEGIN { print "Hello!  I m running in the middle of compiling.\n" }

An eval with a string argument is a way of getting a bit of compile time during 

runtime. For example, this code will be compiled and run after the program is in 

runtime, which allows the code to be built during runtime:

eval "print  Hello!  I m compiling in the middle of running.\n";

Since use is just a way of doing a require operation and an import operation at 

compile time, use can be defined in terms of require using BEGIN:

BEGIN { require File::Find; import File::Find; }

And require can be defined in terms of use with eval:

eval "use File::Find ();";

The pair of parenthesis after File::Find tells use not to import any symbols, which 

emulates how require works.

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