When the day (and time) is not involved, your question does not need Date::Calc or anything fancy.

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

or as sub

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);
[download]