Before going on, I'd like to know if you realize that

my $file1 = <<END_FILE2;
abc
END_FILE2
[download]

is just another way of writing

my $file1 = "abc\n";
[download]

how do you open the heredoc using the 2 arguments of "open"?

Neither know anything about heredocs. Your request to open a heredoc is nonsense.

As for the ability to create file handles that read from or write to a scalar, the 2-arg version can't do that.