From PerlNet

Contents 1 require 2 use 3 eval() 4 do()

require

require VERSION

demands that the Perl binary that the source code is run under is at least the version specified or greater

require BARE::PACKAGE::NAME

• looks for the package in the paths indicated in @INC.
• does not introduce any of the symbols from the package into the current namepace.

   require "PACKAGE::NAME";

or

    $class = 'PACKAGE::NAME";
   require $class;

looks for PACKAGE::NAME in the paths in @INC and complains at compile time.

use

use BARE::PACKAGE::NAME

• does similar to require but imports symbols

indeed:

   use MODULE list;

is the same as

   BEGIN { require MODULE; import MODULE list; }

or

   require MODULE

is similar to

   use MODULE ();

• The empty list at the end stops symbols from being imported.
• if the package cannot be found error generated at compile time.
• As I understand it Perl looks for all the use statements at compile time and brings them in then whereas require happens at runtime
• You can also specify the lowest version of a module that can be used.

use MODULE version;

eval()

eval BLOCK

• BLOCK is a {}
• trap runtime errors that would prove fatal.
• If a trappable error occurs then eval returns undef.
• syntax checked at compile time

eval expr

• code is instead parsed at runtime.
• syntax errors are caught and appear in $@.
• executed in the context of the program that is executing the eval expression so you can see lexical variables (cf do())

do()

do FILE;

• FILE is a string that is a filename
• do causes the execution of the script.
• HOWEVER the code in the do() does not have access to the lexicals that are in the Perl code executing the do statement.
• what is returned from the do function is the last thing that is evaluated.
• See User:PJF post about using do() for configuration.