use Time::Local qw( timelocal_nocheck );

my ($day, $month, $year);

my $now = time;
($day, $month, $year) = (localtime($now))[3, 4, 5];

$month -= 6;

my $new_time = timelocal_nocheck(0, 0, 0, ,$day, $month, $year);
($day, $month, $year) = (localtime($new_time))[3, 4, 5];

printf("%04d-%02d-%02d\n", $year, $month, $day);