my %handler = (
    HEADER => sub { ... },
    TITLE  => sub { ... },
    COMPND => sub { ... },
);

my ($tag, $text) = ("")x2;
while(<>) {
    chomp;
    my ($curr_tag, $curr_text) = split /\s+/, $_, 2;
    if($curr_tag ne $prev_tag) {
        $handler{$tag}->($tag, $text) if exists $handler{$tag};
        # complain_about_unknown() if not exists $handler{$tag}; ?
        ($tag, $text) = ($curr_tag, "");
    }
    else {
        my $curr_linenr;
        ($curr_linenr, $curr_text) = split /\s+/, $curr_text, 2;
        # perform validation on line nr here?
    }
    $text .= " " . $curr_text;
}

##</code><code>##

my %record;

my %handler = (
    # ...
    TITLE => sub { $record{TITLE} = $_[1] unless exists $record{TITLE} },
    # ...
);

##</code><code>##

my $curr_rec = 0;
my @record;

my %handler = (
    # ...
    HEADER => sub { ++$curr_rec },
    TITLE  => sub {
        $record[$curr_rec]->{TITLE} = $_[1]
            unless exists $record[$curr_rec]->{TITLE}
    },
    # ...
);