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 };
}

##</code><code>##

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