Here is a way to do it without procmail using Email::Filter and a sendmail interface such as Email::Sender.

In ~/.forward

| filter_mail

In ~/bin/filter_mail

use Email::Filter;
my $mail = Email::Filter->new(emergency => "~/emergency_mbox");
$mail->accept("$ENV{HOME}/Mail/new") unless $mail->subject =~ /magic_k
+eyword/; # usually exit here
send_mail();

sub send_mail {

# see docs for Email::Sender

}
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