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

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

say for previous_month (10, 2018);