What is the Difference between .pm(perl module) and .pl(perl script) file?

Question

Difference between .pm(perl module) and .pl(perl script) file?

Plz also tell me why we return 1 from file. if return 2 or anything else, its not generating any error, so why we return 1 from Perl module??

Answer 1

A .pl is a single script.

In .pm (Perl Module) you have functions that you can use from other Perl scripts:

A Perl module is a self-contained piece of Perl code that can be used by a Perl program or by other Perl modules. It is conceptually similar to a C link library, or a C++ class.

Answer 2

I guess you replied it your self but to clarify it this could be a very nice reading perldoc perlmod

Perl Modules:

A module is just a set of related functions in a library file, i.e., a Perl package with the same name as the file. It is specifically designed to be reusable by other modules or programs. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it, or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicitly exporting anything. Or it can do a little of both.

In the Perl module, you return 1 at the end so that the require succeeds. You can read more in perldoc perltoot.

Answer 3

At the very core, the file extension you use makes no difference as to how perl interprets those files.

However, putting modules in .pm files following a certain directory structure that follows the package name provides a convenience. So, if you have a module Example::Plot::FourD and you put it in a directory Example/Plot/FourD.pm in path in your @INC, then use and require will do the right thing when given the package name as in use Example::Plot::FourD.

The file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with 1; unless you're sure it'll return true otherwise. But it's better just to put the 1;, in case you add more statements.

If EXPR is a bareword, the require assumes a ".pm" extension and replaces "::" with "/" in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.

All use does is to figure out the filename from the package name provided, require it in a BEGIN block and invoke import on the package. There is nothing preventing you from not using use but taking those steps manually.

For example, below I put the Example::Plot::FourD package in a file called t.pl, loaded it in a script in file s.pl.

C:\Temp> cat t.pl
package Example::Plot::FourD;

use strict; use warnings;

sub new { bless {} => shift }

sub something { print "something\n" }

"Example::Plot::FourD"

C:\Temp> cat s.pl
#!/usr/bin/perl
use strict; use warnings;

BEGIN {
    require 't.pl';
}

my $p = Example::Plot::FourD->new;
$p->something;


C:\Temp> s
something

This example shows that module files do not have to end in 1, any true value will do.