The first Foo is the name of a package, as in...

  package Foo;

  sub bar {
    print "Inside the package, I am called 'bar'\n";
    print "But outside the package, I am called Foo::bar\n";
  }
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The second "Foo" is just a string scalar.

I really think you need to read perlboot from start to finish.

In that example, the sub can be called as Foo::bar() regardless whether you are inside or outside the package. It's just that inside the package, the sub can also be called as bar().