2011-04-28 13:25:47 INFO [main:114] <Message><Tag attribute="value">Answer</Tag></Message>
2011-04-28 13:45:12 DEBUG [Populate::List:31] <Message><Tag attribute="value">Answer</Tag></Message>

##</code><code>##

while (<$fh>) {
    chomp;
    my ($date, $time, $log_lvl, $trace, $xml) = split ' ', $_, 5;
}

##</code><code>##

my $twig = XML::Twig->new();
while (<$fh>) {
    chomp;
    my ($date, $time, $log_lvl, $trace, $xml) = split ' ', $_, 5;
    my %data_structure;
    $twig->parse($xml);
    # Build up %data_structure using $twig
}

##</code><code>##

while (<$fh>) {
    chomp;
    my ($date, $time, $log_lvl, $trace, $xml) = split ' ', $_, 5;
    my $data_structure = extract_data($xml);
}

sub extract_data {
    my ($xml) = @_;
    my $data = {};
    my $twig = XML::Twig->new(
        twig_handlers => {
            Message => sub { handle_message(@_, $data) }
        }
    );
    $twig->parse($xml);
    return $data;
}

sub handle_message {
    # ...
}