$ perl -wE'my($m,$y)=(localtime)[4,5];--$m<0 and($m,$y)=($m+12,$y--);say $m+1,"-",$y+1900;'
9-2018

##</code><code>##

sub previous_month {
    my ($month, $year) = @_; # month = 1..12
    if (--$month < 1) {
        $month = 12;
        $year--;
        }
    ($month, $year);
    } # previous_month

say for previous_month (10, 2018);