$VAR => {
    City-Center1=> {
        Data-items for first combination  }
    City-Center2=> {
        Data-items for second combination }
...
}
[download]

In Pseudo-Code (because I haven't run it through the interpreter) --

my %city_center_hash;

# Loading the data structure --
while <DATA> {
   ($variables) = split data line
   my $this_key = "$o_city|$o_tcenter"; # select an
                                        # appropriate
                                        # separater
   $city_center_hash{$this_key} = {
       data_item1 => variable1,
       data_item2 => variable2,
#      ...
       data_itemN => last_variable,
    };
}   
   
# ....
# ....

# Retrieve the data from the data-structure

foreach $cc_combo (sort keys %city_center_hash) {
 
    my ($this_city, $this_center) = split /\|/, $cc_combo;

    my $these_data = $city_center_hash{$cc_combo};

    my $variable_item1 = $these_data -> {data_item1};

    # Do something with the names and the first 
    # data variable
    # etc ....
}
[download]

  push @{$testcenters{$o_city}, $o_tcenter;
[download]

for my $city (sort keys %testcenters) {
  for my $center (sort @{$testcenters{$city}}) {
    print "$city: $testcenters<br>";
  }
}
[download]

I'm not even sure you need the %testcenters hash (at least not for sorting purposes) because it sounds like you want to sort the test centers by city and for that all you need is the %centercity hash that you've already got. From your code it looks like you've got a series of "parallel hashes" all keyed on the test center name which is your unique identifier. You don't need to change this structure to use something more complicated (like the other fine suggestions given so far) if you don't want to.

Assuming my pile of assumptions are correct, you want something like this:

my @test_centers_sorted_by_city_then_name = sort { $centercity{$a} cmp
+ $centercity{$b} || $a cmp $b } keys %centercity;

for my $center (@test_centers_sorted_by_city_then_name) {
   print "$center<br>";               # output test center name
   print "$centercity{$center}<br>";  # output city where test center 
+is located
   # ... output other stuff too
}
[download]

Again, my apologies for a very poorly formed question, but this is exactly the solution I was looking for. And I actually understand it, which is a bonus. Its what I'd been trying to figure out for quite awhile. Many thanks!

---

I learn more and more about less and less until eventually I know everything about nothing.

Why not give us three lines of data that show the problem, and while you are at it collapse down to the minimum number of fields to show the issue - probably 2.

You don't show us byString.

You really, really should add use strict; use warnings; to every script you write or maintain.

Currently because your variable list is so long and doesn't contain any spaces you node renders about three screen widths wide. It is never good practice to create lines that are longer than can be viewed without scrolling, and good practice generally calls for lines that are fewer than 80 characters wide.

A few lines of example data would be more useful.