Perl - Global variables available in all included scripts and modules?

Question

So lets say I have a main.pl script and in that script I need to declare some variables (any kind of variable constant or normal) and those variables need to be

Answer 1

You can declare global variables with the our keyword:

our $var = 42;

Each global variable has a fully qualified name which can be used to access it from anywhere. The full name is the package name plus the variable name. If you haven't yet declared a package at that point, you are in package main, which can be shortened to a leading ::. So the above variable has the names

$var       # inside package main
$main::var # This is most obvious
$::var     # This may be a good compromise

If we had used another package, the prefix would change, e.g.

package Foo;
our $bar = "baz";
# $Foo::bar from anywhere,
# or even $::Foo::bar or $main::Foo::bar

If we want to use a variable without the prefix, but under other packages, we have to export it. This is usually done by subclassing Exporter, see @Davids answer. However, this can only provide variables from packages that are being used, not the other way round. E.g.

Foo.pm:

package Foo;
use strict; use warnings;
use parent 'Exporter'; # imports and subclasses Exporter

our $var = 42;
our $not_exported = "don't look at me";

our @EXPORT = qw($var); # put stuff here you want to export
# put vars into @EXPORT_OK that will be exported on request

1;

script.pl:

#!/usr/bin/perl
# this is implicitly package main
use Foo; # imports $var

print "var = $var\n"; # access the variable without prefix
print "$Foo::not_exported\n"; # access non-exported var with full name

Lexical variables (declared with my) don't have globally unique names and can't be accessed outside their static scope. They also can't be used with Exporter.