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You can use the global hash %_SHARED to store data, including code references, between function calls for the lifetime of the current session.
Here is a simple example for shared data:
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');
Here is a slightly more complicated example using a code reference:
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 $$
    my $text_to_quote = shift;
    my $qfunc = $_SHARED{myquote};
    return &$qfunc($text_to_quote);
$$ LANGUAGE plperl;(You could have replaced the above with the one-liner return $_SHARED{myquote}->($_[0]); at the expense of readability.)