The "do" has nothing to do with it. It's all magic of the eval $string construct.

Try this, and you'll see the same effect:

my $string = '$data = "foo"';
{
    my $data;
    eval $string;
    print $data;
}
[download]

In both perls I tested this in (5.6.1 and 5.8.3), this prints "foo".

The do BLOCK only serves to locally undef $/, and thus, read the entire file into the string. The result of it, is just a string, that then gets evalled.

You are right that it's a remarkable difference with require and use, that are indeed somewhat similar, as is do STRING; but that's a different kind of do, unrelated to this here, which is do BLOCK.