Siesta, some assembly required

 package Siesta::Plugin::SubjectTag;
 use strict;
 use Siesta::Plugin;
 use base 'Siesta::Plugin';

 sub description {
     'add [list.id] to subject lines';
 }

 sub process {
     my $self = shift;
     my $mail = shift;

     my $subject = $mail->subject || 'no subject';
     my $list_name = $self->list->name;
     $mail->subject("[$list_name] $subject")
       unless $subject =~ /\[$list_name\]/;
     return;
 }

 1;
 __END__