my $log = ...;

...

# Define a file appender when you need it
my $file_appender = Log::Log4perl::Appender->new(
                      "Log::Log4perl::Appender::File",
                      name      => "filelog",
                      filename  => "/tmp/my.log")


$file_appender->log(message => "Log me\n");
$file_appender->file_switch('/tmp/mynew.log');

$log->add_appender($file_appender);
$log->log($TRACE, "...");