Variables declared with our create a package variable and a lexically scoped variable which is an alias to that package variable, visible through the entire scope (file or block) even spanning packages:

# file foo.pl
use strict;
our $foo; # that's $main::foo

{
    package Foo;
    our $foo = "foo"; # package variable $Foo::foo created
    print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n"
+;

    package Bar;
    print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n"
+;
}
print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n";
print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$Foo::foo is '$Foo::f
+oo'\n";
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# file bar.pl
use strict;

{
    package Bar;
    our $foo; # package variable $Bar::foo created
    print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n"
+;
} # end of scope

package Foo;
our $foo; # package variable $Foo::foo initialized in 'foo.pl'
print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n";

package Bar;
print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n";
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#!/usr/bin/perl

use strict;
our $foo = 'bar'; # package variale main::foo
require 'foo.pl';
require 'bar.pl';

print __FILE__,' ',__LINE__,' ',__PACKAGE__,":: \$foo is '$foo'\n";
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Running main.pl yields

foo.pl 8 Foo:: $foo is 'foo'
foo.pl 11 Bar:: $foo is 'foo'
foo.pl 13 main:: $foo is 'bar'
foo.pl 14 main:: $Foo::foo is 'foo'
bar.pl 7 Bar:: $foo is ''
bar.pl 12 Foo:: $foo is 'foo'
bar.pl 15 Bar:: $foo is 'foo'
main.pl 8 main:: $foo is 'bar'
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updated as per JavaFan's comment below. Of course there's only one variable and it's alias in the current scope.