my %sections;
while (my $row = $sth->fetchrow_arrayref) {
    my ($section, $year, $month, $value) = @$row{qw/ Bereich Jahr Monat Wert /};
    $sections{$section}->{$year}->{$month} = $value;
}

for my $section (sort keys %sections) {
    print "$section |";
    my $years = $sections{$section};
    for my $year (sort keys %$years) {
        print " $year|";
        my $months = $years->{$year};
        my @values = map {
            defined $months->{$_} ? $months->{$_} : ""
        } 1 .. 12;
        local $" = " | ";
        say "@values";
    }
}