The header lines have to be flush-left (i.e., preceded by newlines). You've got white space in front of them. Try:

print SENDMAIL qq^
From: Monitor <root\@host.domain.com>
To: Pen Quin <penguinfuz\@another-domain.com>
Subject: $hostname Monitoring Script

Hello, I am the monitoring script for $hostname.
^;
[download]

BTW, the here-doc is just great for this kind of stuff:

print SENDMAIL <<EOM;
From: Monitor <root\@host.domain.com>
To: Pen Quin <penguinfuz\@another-domain.com>
Subject: $hostname Monitoring Script

Hello, I am the monitoring script for $hostname.
EOM
[download]

Although it has the same white-space-preserving behavior. You can get around with some contortions, but it's a lot easier to just put everything flush-left.

HTH <code