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15th chunk of `doc/src/sgml/plperl.sgml`
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 <application>PL/PerlU</application> functions, or <application>PL/Perl</application> functions
    that are all called by the same SQL role.
   </para>
  </note>

 </sect1>

 <sect1 id="plperl-triggers">
  <title>PL/Perl Triggers</title>

  <para>
   PL/Perl can be used to write trigger functions.  In a trigger function,
   the hash reference <varname>$_TD</varname> contains information about the
   current trigger event. <varname>$_TD</varname> is a global variable,
   which gets a separate local value for each invocation of the trigger.
   The fields of the <varname>$_TD</varname> hash reference are:

   <variablelist>
    <varlistentry>
     <term><literal>$_TD-&gt;{new}{foo}</literal></term>
     <listitem>
      <para>
       <literal>NEW</literal> value of column <literal>foo</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{old}{foo}</literal></term>
     <listitem>
      <para>
       <literal>OLD</literal> value of column <literal>foo</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{name}</literal></term>
     <listitem>
      <para>
       Name of the trigger being called
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{event}</literal></term>
     <listitem>
      <para>
       Trigger event: <literal>INSERT</literal>, <literal>UPDATE</literal>,
       <literal>DELETE</literal>, <literal>TRUNCATE</literal>, or <literal>UNKNOWN</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{when}</literal></term>
     <listitem>
      <para>
       When the trigger was called: <literal>BEFORE</literal>,
       <literal>AFTER</literal>, <literal>INSTEAD OF</literal>, or
       <literal>UNKNOWN</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{level}</literal></term>
     <listitem>
      <para>
       The trigger level: <literal>ROW</literal>, <literal>STATEMENT</literal>, or <literal>UNKNOWN</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{relid}</literal></term>
     <listitem>
      <para>
       OID of the table on which the trigger fired
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{table_name}</literal></term>
     <listitem>
      <para>
       Name of the table on which the trigger fired
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{relname}</literal></term>
     <listitem>
      <para>
       Name of the table on which the trigger fired. This has been deprecated,
       and could be removed in a future release.
       Please use $_TD-&gt;{table_name} instead.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{table_schema}</literal></term>
     <listitem>
      <para>
       Name of the schema in which the table on which the trigger fired, is
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{argc}</literal></term>
     <listitem>
      <para>
       Number of arguments of the trigger function
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>@{$_TD-&gt;{args}}</literal></term>
     <listitem>
      <para>
       Arguments of the trigger function.  Does not exist if <literal>$_TD-&gt;{argc}</literal> is 0.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

  <para>
   Row-level triggers can return one of the following:

   <variablelist>
    <varlistentry>
     <term><literal>return;</literal></term>
     <listitem>
      <para>
       Execute the operation
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>"SKIP"</literal></term>
     <listitem>
      <para>
Title: PL/Perl Triggers in PostgreSQL
Summary
This section explains how to use PL/Perl for writing trigger functions in PostgreSQL. It describes the $_TD hash reference, which contains information about the current trigger event. The $_TD hash includes details such as the NEW and OLD values of columns, trigger name, event type (INSERT, UPDATE, DELETE, etc.), when the trigger was called (BEFORE, AFTER, etc.), trigger level (ROW, STATEMENT), table information, and trigger function arguments. The section also outlines the possible return values for row-level triggers, including executing the operation, skipping the operation, or modifying the row to be inserted or updated.