I'm puzzled as to why you use a hash with December associated with zero and a modulo operation. That will only work for the first decrement of a month. To go back another month you would need a different hash with November associated with zero, and so on and so forth. I think it might be better to have a single hash with zero for January to eleven for December (as returned by localtime) and a simple algorith for decrementing the month value, something like

my %months = (
   0  => q{Jan},
   1  => q{Feb},
   2  => q{Mar},
   3  => q{Apr},
   4  => q{May},
   5  => q{Jun},
   6  => q{Jul},
   7  => q{Aug},
   8  => q{Sep},
   9  => q{Oct},
   10 => q{Nov},
   11 => q{Dec},
   );
my $month = (localtime)[4];
$month --;
$month = 11 if $month < 0;
[download]

This way you can keep decrementing the month until the cows come home.

Cheers,

JohnGG