foreach $month (%keys %ManagerInCharge) {
   my $month_hash = $ManagerInCharge{$month};
   my @month_array = @$month_hash{
      sort { $a <=> $b } keys %$month_hash
   };
   my $i;
   $ManagerInCharge{$month} = { map { ++$i => $_ } @month_array };
}
[download]

Or if you want to switch to a HoA,

foreach $month (%keys %ManagerInCharge) {
   my $month_hash = $ManagerInCharge{$month};
   my @month_array = @$month_hash{
      sort { $a <=> $b } keys %$month_hash
   };
   $ManagerInCharge{$month} = \@month_array;
}
[download]