Try this on for size:

#!/usr/bin/perl -w

use Carp;

my %people = (
    Bob => { age => 60, town => 'Harrisburg' },
    Joe => { age => 55, town => 'Calgary' },
    Sue => { age => 40, town => 'Houston' },
    Jim => { age => 42, town => 'Dresden' },
    Ann => { age => 47, town => 'Los Angeles' },
);

my %data_types = (
    age => 'numeric',
    town => 'text',
    # etc...
);

sub sort_people_by {
    my ($people,$sortby,$dir) = @_;

    my $dtype = $data_types{$sortby};

    if(!$dtype) {
        croak "No data type defined for $sortby";
    }
    elsif($dtype eq 'numeric') {
        if($dir eq 'ASC') {
            return sort { $people->{$a}->{$sortby}
                      <=> $people->{$b}->{$sortby} }
                      keys %$people;
        }
        else {
            return sort { $people->{$b}->{$sortby}
                      <=> $people->{$a}->{$sortby} }
                      keys %$people;
        }
    }
    elsif($dtype eq 'text') {
        if($dir eq 'ASC') {
            return sort { $people->{$a}->{$sortby}
                      cmp $people->{$b}->{$sortby} }
                      keys %$people;
        }
        else {
            return sort { $people->{$b}->{$sortby}
                      cmp $people->{$a}->{$sortby} }
                      keys %$people;
        }
    }
    else {
        croak "Data type $dtype unknown";
    }
}

sub print_people_sorted_by {
    my ($people,$sortby,$dir) = @_;

    foreach my $person
              (sort_people_by($people,$sortby,$dir)) {
        print "$person - $people{$person}{$sortby}\n";
    }
}

print_people_sorted_by(\%people,'age','ASC');
print_people_sorted_by(\%people,'town','DESC');
[download]

Updated style to be closer to PBP, since I need the practice anyways - old habits die hard :)