Minor counter-note: the synopsis shows both arrayref and scalar:

    header_str => [
        From => 'casey@geeknest.com',
        To => [ 'user1@host.com', 'Name <user2@host.com>' ],
        Cc => Email::Address::XS->new("Display Name \N{U+1F600}", 'use
+r@example.com'),
    ],
...
    header_str => [
        From => 'my@address',
        To   => 'your@address',
    ],
[download]

so either are presumably allowed. The important thing is that the From and the Subject are never shown as arrayref (and it doesn't make sense for more than one sender or more than one subject line)