PL/Perl</secondary>
      </indexterm>
     </term>
     <listitem>
      <para>
        Returns the contents of the referenced array as a string in array constructor format
        (see <xref linkend="sql-syntax-array-constructors"/>).
        Individual values are quoted using <function>quote_nullable</function>.
        Returns the argument value, quoted using <function>quote_nullable</function>,
        if it's not a reference to an array.
        </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <literal><function>looks_like_number(<replaceable>string</replaceable>)</function></literal>
      <indexterm>
       <primary>looks_like_number</primary>
       <secondary>in PL/Perl</secondary>
      </indexterm>
     </term>
     <listitem>
      <para>
        Returns a true value if the content of the given string looks like a
        number, according to Perl, returns false otherwise.
        Returns undef if the argument is undef.  Leading and trailing space is
        ignored. <literal>Inf</literal> and <literal>Infinity</literal> are regarded as numbers.
        </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <literal><function>is_array_ref(<replaceable>argument</replaceable>)</function></literal>
      <indexterm>
       <primary>is_array_ref</primary>
       <secondary>in PL/Perl</secondary>
      </indexterm>
     </term>
     <listitem>
      <para>
        Returns a true value if the given argument may be treated as an
        array reference, that is, if ref of the argument is <literal>ARRAY</literal> or
        <literal>PostgreSQL::InServer::ARRAY</literal>.  Returns false otherwise.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </sect2>
 </sect1>

 <sect1 id="plperl-global">
  <title>Global Values in PL/Perl</title>

  <para>
    You can use the global hash <varname>%_SHARED</varname> to store
    data, including code references, between function calls for the
    lifetime of the current session.
  </para>

  <para>
    Here is a simple example for shared data:
<programlisting>
CREATE OR REPLACE FUNCTION set_var(name text, val text) RETURNS text AS $$
    if ($_SHARED{$_[0]} = $_[1]) {
        return 'ok';
    } else {
        return "cannot set shared variable $_[0] to $_[1]";
    }
$$ LANGUAGE plperl;

CREATE OR REPLACE FUNCTION get_var(name text) RETURNS text AS $$
    return $_SHARED{$_[0]};
$$ LANGUAGE plperl;

SELECT set_var('sample', 'Hello, PL/Perl!  How''s tricks?');
SELECT get_var('sample');
</programlisting>
  </para>

  <para>
   Here is a slightly more complicated example using a code reference:

<programlisting>
CREATE OR REPLACE FUNCTION myfuncs() RETURNS void AS $$
    $_SHARED{myquote} = sub {
        my $arg = shift;
        $arg =~ s/(['\\])/\\$1/g;
        return "'$arg'";
    };
$$ LANGUAGE plperl;

SELECT myfuncs(); /* initializes the function */

/* Set up a function that uses the quote function */

CREATE OR REPLACE FUNCTION use_quote(TEXT) RETURNS text AS $$
    my $text_to_quote = shift;
    my $qfunc = $_SHARED{myquote};
    return &amp;$qfunc($text_to_quote);
$$ LANGUAGE plperl;
</programlisting>

   (You could have replaced the above with the one-liner
   <literal>return $_SHARED{myquote}-&gt;($_[0]);</literal>
   at the expense of readability.)
  </para>

  <para>
   For security reasons, PL/Perl executes functions called by any one SQL role
   in a separate Perl interpreter for that role.  This prevents accidental or
   malicious interference by one user with the behavior of another user's
   PL/Perl functions.  Each such interpreter has its own value of the
   <varname>%_SHARED</varname> variable and other global state.  Thus, two
   PL/Perl functions will share the same value of <varname>%_SHARED</varname>
   if and only if they are executed by the same SQL role.  In an application
   wherein a single session executes code under multiple SQL roles (via
   <literal>SECURITY DEFINER</literal>