if ($Month =0)
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should be

if ($Month ==0)
[download]

But I'd do it with an array instead, something like:

my @Month_name = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
my $New_mo = $Month_name[$Month]
[download]

---
perl -le'$$=substr(%{*::}->{_},2,1);print+(map{$$++for(1..$_);$$}(2,5,10)),$|'