This month => 03 or March
last month => 02 or February
Month before last => 01 or January
Month before => 12 or December (of last year)

This code is not working:

my $tm = localtime;
my $this_month   = sprintf("%04d%02d?", $tm->year+1900, ($tm->mon)+1);
my $last_month   = sprintf("%04d%02d?", $tm->year+1900, ($tm->mon));
my $last_month_1 = sprintf("%04d%02d?", $tm->year+1900, ($tm->mon)-1);
my $last_month_2 = sprintf("%04d%02d?", $tm->year+1900, ($tm->mon)-2);