use List::Util qw/min max/;

my %results = map {
    my $year_ref = $data{$_};
    my $min = min values %$year_ref;
    my $max = max values %$year_ref;
    my @min_months = grep { $year_ref->{$_} == $min } keys %$year_ref;
    my @max_months = grep { $year_ref->{$_} == $max } keys %$year_ref;

    $_ => {
        "min"           => $min,
        "max"           => $max,
        "min_months"    => \@min_months,
        "max_months"    => \@max_months
    }
} keys %data;