use POSIX qw( floor strftime );

my $monthsback = 3;   # 0=this month -1=last, -2, ...

my ($day,$mon,$year) = (localtime())[3,4,5];
$mon  -= $monthsback;
$year += floor($mon / 12);
$mon  = $mon % 12;

print(strftime("%Y-%m", 0,0,0, $day,$mon,$year), "\n");