P5-ho-trigger is a project mainly written in Perl, it's free.
Adapt Class::Trigger for HO library.
HO-Trigger
INSTALLATION
To install this module, run the following commands:
perl Makefile.PL
make
make test
make installCOPYRIGHT AND LICENSE
Copyright (C) 2009 Sebastian Knapp
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.