I would build the %month-hash like this:

my %month;
for my $language (keys %month_names) {
  my $n=0;
  $month{$_} = $month{uc $_} = $month{lc $_} = $n++ for @{$month_names
+{$language}};
}
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This way you don't need the @month_numbers-array at all and for my taste it it clearer.