A few comments to your Perl code,

sub main {
  print "Hello, world!\n";
}

unless (caller) {
  main;
}
[download]

This code is a weird formulation because the file that contains main() will always be executed as a main program? Normal practice would be to do away with this extra level of indentation implied by the subroutine main and just start writing the "main" code. The unless (caller){} does nothing useful. You could have just put a simple main(); instead to call the sub main.

I attach demomain.pl and demo.pm below.
Note that demomain.pl could have been called "demo.pl", but I didn't want to confuse you. Anyway note that you can have a .pm file of the same name as a .pl file.

In my demo.pm file, I use caller(), test() if !caller;. If demo.pm is being run as a main program, test() will execute. If demo.pm is being "used" by another program, say by demomain.pl, all of the top-level code in demo.pm will run, but test() will not run because Perl knows that demo.pm is not being run as a main program. There is no need to so something similar in your main.pl program because your main is always a main program!

This provides a very "lightweight" test framework. There are such things as .t files which are used in more complicated situations.

It is possible to split a main program across multiple files, i.e., same package in multiple files. I don't demo that because I think it is a very bad idea.

#!/usr/bin/perl
# File: demo.pl

use strict;
use warnings;
$|=1; # turn off buffering

use Demo qw(example);

# test() is not exported by Demo.pl
# this the error produced...
# Undefined subroutine &main::test called at C:\Projects_Perl\testing\
+demo.pl line 7.
# my $x = test();


my $y = Demo::test();  #this is ok , Fully qualified name
print "test() returned $y\n";

__END__
Prints:
top level in Demo.pm
this is the test subroutine!
test() returned 1
[download]

#!/usr/bin/perl
# File: Demo.pm

use strict;
use warnings;
package Demo;
use vars qw(@ISA @EXPORT @EXPORT_OK %EXPORT_TAGS $VERSION);
use Exporter;
our $VERSION=1.01;
our @ISA = qw(Exporter);
our @EXPORT    = qw();
our @EXPORT_OK = qw(example);
     
our $DEBUG =0;
test() if !caller; # runs test() if run as a "main program"

sub test { print "this is the test subroutine!\n"; return 1;}

sub example {return "this is an example"};

print "top level in Demo.pm\n";


1;
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.