a primer on trigger?

What I want to do is "trigger" a
system call when a table is modified
in any way.  But it appears
that trigger idea in postgres
is not capable.  Can any one
confirm if that is the case?

What I want to do is run an
external perl script which
needs the newly modified row
as input.

My web server modifies the database which
is on a different physical machine.
Currently I'm doing it via
a complicated mechanism of sending
an email to an account at database
machine, catching it using procmail
and then perl script performing
the action.  Perl script needs
the newly modified row as input
and then it makes system call.

Thanks in advance