As I understand it, at end of enclosing block, the package ends, and you are in package main.

No. You are back in the package you came from:

package Foo {
    print "(1) package " . __PACKAGE__ . $/;
    package Bar {
        print "(2) package " . __PACKAGE__ . $/;
    }
    print "(3) package " . __PACKAGE__ . $/;
}
print "(4) package " . __PACKAGE__ . $/;
__END__
(1) package Foo
(2) package Bar
(3) package Foo
(4) package main
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