The code provided by moritz above works, but does not sort the output as it seems you wanted. To have the output sorted, you could add a sort sub to his code as in the following:

for my $k (sort keys %ManagerInCharge){
    print "Month: $k\n";
    my @keys = sort {$a <=> $b} keys %{$ManagerInCharge{$k}};
    print "Managers: ", join(",", @keys), "\n";
}
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