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    <p>Standard practice is to use <code>use</code> most of the time, <code>require</code> occasionally, and <code>do</code> rarely.</p> <p><code>do 'file'</code> will execute <code>file</code> as a Perl script. It's almost like calling <code>eval</code> on the contents of the file; if you <code>do</code> the same file multiple times (e.g. in a loop) it will be parsed and evaluated each time which is unlikely to be what you want. The difference between <code>do</code> and <code>eval</code> is that <code>do</code> can't see lexical variables in the enclosing scope, which makes it safer. <code>do</code> is occasionally useful for simple tasks like processing a configuration file that's written in the form of Perl code.</p> <p><code>require 'file'</code> is like <code>do 'file'</code> except that it will only parse any particular file one time and will raise an exception if something goes wrong. (e.g. the file can't be found, it contains a syntax error, etc.) The automatic error checking makes it a good replacement for <code>do 'file'</code> but it's still only suited for the same simple uses.</p> <p>The <code>do 'file'</code> and <code>require 'file'</code> forms are carryovers from days long past when the *.pl file extension meant "Perl Library." The modern way of reusing code in Perl is to organize it into modules. Calling something a "module" instead of a "library" is just semantics, but the words mean distinctly different things in Perl culture. A library is just a collection of subroutines; a module provides a namespace, making it far more suitable for reuse.</p> <p><code>use Module</code> is the normal way of using code from a module. Note that <code>Module</code> is the package name as a bareword and not a quoted string containing a file name. Perl handles the translation from a package name to a file name for you. <code>use</code> statements happen at compile time and throw an exception if they fail. This means that if a module your code depends on isn't available or fails to load the error will be apparent immediately. Additionally, <code>use</code> automatically calls the <code>import()</code> method of the module if it has one which can save you a little typing.</p> <p><code>require Module</code> is like <code>use Module</code> except that it happens at runtime and does not automatically call the module's <code>import()</code> method. Normally you want to use <code>use</code> to fail early and predictably, but sometimes <code>require</code> is better. For example, <code>require</code> can be used to delay the loading of large modules which are only occasionally required or to make a module optional. (i.e. use the module if it's available but fall back on something else or reduce functionality if it isn't.)</p> <p>Strictly speaking, the only difference between <code>require Module</code> and <code>require 'file'</code> is that the first form triggers the automatic translation from a package name like <code>Foo::Bar</code> to a file name like <code>Foo/Bar.pm</code> while the latter form expects a filename to start with. By convention, though, the first form is used for loading modules while the second form is used for loading libraries.</p>
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