You can devise your own header in the "X-mumble" space, or else put it in Comments. Call it, say, X-SourceScript, and fill it in with $0 when you set up headers for the mail.

my $msg = MIME::Lite->new(
    From     =>'me@myhost.com',
    To       =>'you@yourhost.com',
    Cc       =>'some@other.com, some@more.com',
    Subject  => $title,
    X-SourceScript => $0,
    Data     => $message
);
[download]

The MIME::Lite pod has more about other forms of custom headers.