In Perl, what is the difference between a .pm (Per

Question

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

Please also tell me why we return 1 from file. If return 2 or anything else, it's not generating any error, so why do we return 1 from Perl module?

Answer 1

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 a 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.

Answer 2

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.