The lexical variable in your example is in the package scope.

Compare this:

package foo;
our $n = 1;

package main;

print "n = ", $foo::n , $/; # n = 1
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With this:

package foo;
my $n = 1;

package main;

print "n = ", $foo::n , $/; # nothing
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A related topic is the behaviour of closures, which was mentioned by several other monks above. One of the less obvious behaviours of closures can be seen in the following example:

sub foo {
    my $n = 1;
    print $n;
    bar();
    print $n;
    bar();
    sub bar {
        $n++;
    }
}

foo();
foo();

#output 1211
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The inner sub 'bar' keeps a reference to the instance of '$n' that existed during the first call to 'foo'. People frequently run into this problem when they first start using mod_perl with Apache::Registry, as documented here:
mod_perl / Apache::Registry accidental closures