use warnings;
use strict;

use DateTime;

my $dt = DateTime->now;
my %report;

while (<DATA>) {
    chomp;
    my %month_data = split;
    for (keys %month_data){
        $report{$_} += $month_data{$_};
    }
}

my $report_months = 11;

while ($report_months > -1){
    my $now = $dt->clone;
    my $past_month 
      = $now->subtract(months => $report_months)->month_name;

    print exists $report{$past_month}
        ? "$past_month: $report{$past_month}\n"
        : "$past_month: no data available\n";

    $report_months--;
}

__DATA__
March 5 July 4 August 5 October 7
January 3 March 4 April 6 May 5 June 7 July 8 August 10 
September 4 October 9 November 11 December 8
March 2 June 3 August 1 December 7

##</code><code>##

October: 16
November: 11
December: 15
January: 3
February: no data available
March: 11
April: 6
May: 5
June: 10
July: 12
August: 16
September: 4