Custom variables in email header - possible?

Having JSON variables in email header to store data

[tomda]

Sorry if I post in the wrong forum, i couldn't find a thread about email.

I wanted to know if it is possible to have custom variables in email header, JSON formatted, and presumably under that Extra-Header or Extra-parameters.
I really struggle to find information about this.

[csdude55]

Sure, just put the variable before the "Subject" line. Then you can view it in the source.

In Perl, I have this:

open(MAIL,"|$mailprog -t");
 print MAIL "To: [e-mail]\n";
 print MAIL "From: example\@example\n";
 print MAIL "Whatever: some data I want to send\n";
 print MAIL "Subject: Test Email\n\n";

 print MAIL "This is a test";
close(MAIL);