The following two things are roughly equivalent...

Using Scope::Upper...

use 5.010;
use Scope::Upper qw(localize UP);
our $foo = 42;
sub foo {
  localize '$foo', 1, UP;
}
sub bar {
  say "start bar: ", $foo;
  foo();
  say "end bar: ", $foo;
}
say "before bar: ", $foo;
bar();
say "after bar: ", $foo;
[download]

Without Scope::Upper...

use 5.010;
our $foo = 42;
sub bar {
  say "start bar: ", $foo;
  local $foo = 1;
  say "end bar: ", $foo;
}
say "before bar: ", $foo;
bar();
say "after bar: ", $foo;
[download]

So, essentially, what it does is allows you to change the value of a variable in your caller's scope, with the change reverting once the caller scope is finished.