It is a custom (not sure whether/where it is documented), to start custom headers with X-. Email servers and clients don't touch these X-headers. You could add your scriptname like:

Subject: testmail
To: test@test.com
X-Scriptname: /usr/local/bin/myscript.pl

PS: oh, the joy I had adding X-wing or X-mas headers to email ...

PS2: X-headers are actually documented in RFC822 (http://www.faqs.org/rfcs/rfc822.html)