# main_template aus DB lesen
my $statement = "SELECT template FROM template WHERE name='main_template'";
my $sth = $dbh->prepare($statement)          || die $dbh->errstr;
unless( my $rv = $sth->execute ){ die "Konnte main_template nicht aus Datenbank lesen"; }
my $template = HTML::Template->new(arrayref => $sth->fetchrow_arrayref() );
$sth->finish();

# self füttern
$template->param(SELF => $cgi->url(-relative=>1) . '?sid=' . $query->{sid});

# Ausgabe
print $template->output;