PL/Perl can be used to write trigger functions.  In a trigger function,
   the hash reference $_TD contains information about the
   current trigger event. $_TD is a global variable,
   which gets a separate local value for each invocation of the trigger.
   The fields of the $_TD hash reference are:
   
$_TD->{new}{foo}       NEW value of column foo
      
$_TD->{old}{foo}       OLD value of column foo
      
$_TD->{name}Name of the trigger being called
$_TD->{event}       Trigger event: INSERT, UPDATE,
       DELETE, TRUNCATE, or UNKNOWN
      
$_TD->{when}       When the trigger was called: BEFORE,
       AFTER, INSTEAD OF, or
       UNKNOWN
      
$_TD->{level}       The trigger level: ROW, STATEMENT, or UNKNOWN
      
$_TD->{relid}OID of the table on which the trigger fired
$_TD->{table_name}Name of the table on which the trigger fired
$_TD->{relname}Name of the table on which the trigger fired. This has been deprecated, and could be removed in a future release. Please use $_TD->{table_name} instead.
$_TD->{table_schema}Name of the schema in which the table on which the trigger fired, is
$_TD->{argc}Number of arguments of the trigger function
@{$_TD->{args}}       Arguments of the trigger function.  Does not exist if $_TD->{argc} is 0.
      
Row-level triggers can return one of the following:
return;Execute the operation
"SKIP"Don't execute the operation
"MODIFY"       Indicates that the NEW row was modified by
       the trigger function
      
Here is an example of a trigger function, illustrating some of the above:
CREATE TABLE test (
    i int,
    v varchar
);
CREATE OR REPLACE FUNCTION valid_id() RETURNS trigger AS $$
    if (($_TD->{new}{i} >= 100) || ($_TD->{new}{i} <= 0)) {
        return "SKIP";    # skip INSERT/UPDATE command
    } elsif ($_TD->{new}{v} ne "immortal") {
        $_TD->{new}{v} .= "(modified by trigger)";
        return "MODIFY";  # modify row and execute INSERT/UPDATE command
    } else {
        return;           # execute INSERT/UPDATE command
    }
$$ LANGUAGE plperl;
CREATE TRIGGER test_valid_id_trig
    BEFORE INSERT OR UPDATE ON test
    FOR EACH ROW EXECUTE PROCEDURE valid_id();