Run a unix or perl command in a trigger

Hi Postgres users,

I wonder to know if it is possible to launch a Perl program or a unix
comand in a trigger function.
I have tried to do that in a C trigger developed with the SPI function.
But my examples does not work and i don't have any error messages in the
postgres logfile.
So, i ask you your opinion and an example of code if your trigger works.

Regards,
Thierry Missimilly