Interestingly...

use Test::More tests => 1;

# this test fails
is(
  do { package Foo::Bar::; __PACKAGE__ },
  do { package Foo::Bar;   __PACKAGE__ },
);
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So there's at least one place the barewords Foo::Bar:: and Foo::Bar evaluate to different packages.

See also, Perl allows package names consisting entirely of colons.