I find problems with your code that have not been commented on. Your specifically the direction part is a red herring that responders seemed to chase.

The problems with your mysort are that it is single purpose and it only sorts the %This_package::hash and nothing else. It is bad that one must read mysort to know that. The function is also poorly named and is defined at the wrong level. Defined in a logical, or problem domain modelling, sense.

If you can, use the standard style of sorting. This is to use the $a and $b variables to indicate how you are sorting. If you want a reversed sort you use $b $a to show that. This is to define the sort at a lower and more generic level.

I would suggest you use something like:

sub num_or_str {
    no warnings 'numeric'; 
    $_[0] <=> $_[1] || $_[0] cmp $_[1]
}

my @sorted 
  = sort num_or_str( 
            $people{$a}->{age}, 
            $people{$b}->{age} 
         ), keys %people;
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Then you can reverse the order of the arguments to num_or_str to reverse the sort. I consider that idiomatic Perl as below.

If you really need a direction flag variable, you could keep it readable:

my @stuff = qw/ 9 -1 -3 hi 0 bye 5 10 /;

my $descending = 1;
my @ar;

if ( $descending ) {
       @ar = sort { num_or_str($b, $a) } @stuff;
}
else {
       @ar = sort { num_or_str($a, $b) } @stuff;
}
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If you want to structure code more like your sample you should put the function around the sort. That is what I meant about creating the function at the wrong level.

my $sort_by = 'age';
my $descending = 1;
my $database = \%people;

# return an ordered list of copies of keys of a 
# hash structured as described somewhere
# usage: sort_people( \%sort_me, $by_me, $descending )
# Ascending sort is the default.
# If $by_me is not a key that exists then ...
# Maybe an arref should be returned.
sub sort_people {
    my ( $people, $by, $desc) = @_;
    no warnings qw/ numeric uninitialized/;
    if ( $desc) {
        return sort { your_conditions( $b, $a)} keys %$people;
    } else {
        return sort { your_conditions( $a, $b)} keys %$people;
    }
}
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Then this higher level code is easier to understand:

my @sorted_keys = sort_people( \%people, $sort_on, $descending);
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Be well,
rir