When you use our you're accessing package variables without having to explicitly name them. That is, when you write:

package Foo;
our $var;
$var = 10;
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you're actually setting the $Foo::var. The our declaration is lexically scoped. The following example could be of some use:

package Foo;
our $d;               # FIRST
$d = __PACKAGE__;

package Bar;
sub bar {
   print 'in ', __PACKAGE__, '::bar $d is equal to ', "$d\n";
   return;
}
our $d = __PACKAGE__; # SECOND
sub baz {
   print 'in ', __PACKAGE__, '::baz $d is equal to ', "$d\n";
   return;
}

package main;
Bar::bar();
Bar::baz();

__END__

poletti@PolettiX:~/sviluppo/perl$ perl our.pl
in Bar::bar $d is equal to Foo
in Bar::baz $d is equal to Bar
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Bar::bar() "sees" the FIRST declaration, while

Bar::baz()> is in the scope of the second.
<p>In your example, you actually access <c>$Bar::d
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in the anonymous sub you pass to Foo::top(): the fact that it has the same value of $Foo::d depends on the assignment:

   our($d) = @_;
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which makes $Bar::d equal to $Foo::d - ach! :)